Depending on StringProperty's display type, its property widget shows either just the QLineEdit, or additionally a browse button that pops up a QFileDialog for either loading or saving a file, or choosing a directory.

closes #43

* Geometry1DTransferFunctionEditor not correctly drawing image intensity histogram.
* VolumeExplorer not always initializing slice- and raycasting render size properties.

* Fixed RTTI type inference in PropertyWidgetFactory::createWidget()

Implemented a real PropertyWidgetFactory where property widgets can register themselves together with the property type they wrap around. Registration is implemented using CRTP do allow automatic registration across DLL boundaries during static initialization time.
This commit replaces the long list of if-else in PropertyWidgetFactory with a much more generic approach.

Fixed GeometryTransferFunctionEditors trying to initialize their OpenGL contexts from non OpenGL thread.

Removed oblivious tgt::QtContextManager and merged functionality into tgt::GLCanvas and tgt::GlContextManager. Meanwhile, improved OpenGL context initialization and pipeline creation in CampvisApplication.

Added s_propertyAdded and s_propertyRemoved signals to PropertyCollection signaling changes to the property list. PropertyCollectionWidget listens to them and updates the GUI on the fly.

* Support read-out of texel values also for 3D images
* introducing tgt::Texture::texelAsFloat() for 3D images
* Fixed Qt crashes in FloatPropertyWidget caused by changing GUI from non-GUI thread

Completely moved the image data handle and histogram computation from the TF to the TF property. Histogram computation is now thread safe.

Started refactoring the TransferFunctionProperty: The optional ImageData handle will be stored in the TF property instead of the TF itself. Furthermore, the property now supports automatically fitting the TF window to the image data.

fixed PropertyCollectionWidget changing visibility of property widgets from non-Qt thread (now using Qt signals).

Closes Issue #16: Geometry TF editor thread-safety: Reviewed the code + extensive stress testing - fixed a recursive mutex acquisition.

* Moving to Apache 2.0 license
* Updated AUTHORS.txt

 * fixed 1D geometry TF editor
 * introduced OptionProperty to specify gradient computation mode
 * added s_changed signal of properties also being propagated to its shared properties
 * fixed sampling rate compensation when using adaptive sampling in simpleraycaster.frag

Fixed missing/wrong deregistration of GeometryXdTransferFcuntionEditors when their widgets are closed (and hence the GL contexts destroyed)

Work on Issue #44: Removed GLContext interface and merged functionality into GlCanvas. Introducing abstract (Qt-free) tgt::GlContextManager interface, being implemented by tgt::QtContextManager. Further clean up of related code.

Revised event handling system: Got rid of campvis::AbstractEventHandler and replaced with functionality of tgt::EventListener and tgt::EventHandler

FloatPropertyWidget and VecPropertyWidget configure their adjusters to
display as many decimals as their properties have significant decimal
places. Moreover, they connect to the s_decimalsChanged signal in order
to reconfigure their adjusters when the number of significant decimal
places of their properties changes.

Floating point properties were previously typedef'd to specific
instantiations of NumericProperty. Now that we have
FloatingPointProperty, which extends NumericProperty to control how many
decimal places should be shown when displaying the property's value, all
typedefs for floating point properties have been updated to point to it
instead.

As a result, many processors and pipelines needed to have their includes
and/or constructors fixed to import and work with the new typedefs.

A new attribute has been added to all numeric properties: step value. It
determines the value of a single increment/decrement that numeric
property widgets use when their associated properties are modified using
sliders or spin boxes (users are still able to type any valid property
value in text edits).

Numerous processors and pipelines had to be updated to work with the
changed NumericProperty interface. However, choosing a well-suited step
value for each property can make it easier for users to modify the
property — using one step value for all properties leads to cases where
it is either too small, causing users to go through property values
which don't cause any visible change, or too big, making it difficult to
quickly determine the right property value.

Previously, widgets for int and vector<int> properties used only spin
boxes to display and let users modify the values of their associated
property.

Now that we have IntAdjusterWidget, IntPropertyWidget and all
IVecPropertyWidgets have been updated to use it to make it possible to
change the values of their components with not only a spin box, but also
a slider.

DoubleAdjusterWidget used to inherit directly from QWidget. Rebasing it
on top of AbstractAdjusterWidget<double>, however, greatly simplified
its implementation and provided it with several additional API methods
for free.

AbstractPropertyWidget's onPropertyChanged slot is invoked from non-GUI
threads. Previously, it would call updateWidgetFromProperty directly,
which resulted in Qt widgets being accessed from non-GUI threads. This
in turn led to random crashes when properties were modified quickly and
repeatedly from the GUI.

Now we invoke updateWidgetFromProperty via a signal-slot connection with
an internal signal, s_propertyChanged. This makes Qt take care of
queueing slot accesses in the GUI thread for us.

Fixes #36

Previously, property widgets have always had their components displayed
horizontally next to a label with the property name. However, that made
widgets consisting of many components (e.g. widgets for vector
properties) hard to use because their components were squeezed together.

Now AbstractPropertyWidget takes one additional optional argument,
displayBoxed. If it's false, the widget is displayed as before. When
it's true, the widget is laid out vertically in a QGroupBox. For now
only widgets for vector properties set displayBoxed to true.

Previously, widgets for double-valued vector properties used only spin
boxes to display and let users modify the values of their associated
property.

Now that we have DoubleAdjusterWidget, vector property widgets have been
updated to use it to make it possible to change the values of their
components with not only a spin box, but also a slider.

Previously, FloatPropertyWidget used standard spin boxes and sliders to
display and let users modify the value of its associated property.
Consequently, it had to synchronise the states of those widgets on its
own.

Now that we have a separate widget that implements the two ways of
changing double values that FloatPropertyWidget provides, it makes sense
to simplify FloatPropertyWidget's code by making it use
DoubleAdjusterWidget internally.

Each FloatPropertyWidget now gives the user two ways of modifying its
underlying property's value: the slider can be used for quick changes,
the spin box — for precise adjustments.

QtThreadedCanvas used to ignore all repaint events send to it by Qt in
order to avoid problems with threading. As a result, however, it would
become blank when docked/undocked, and display garbage in those of its
regions that some other widgets have moved over.

To fix this, the API of TGT's Painter class had to be slightly modified
and now mirrors that of Canvas with regard to painting-related
operations. The paint method has been made protected; it's supposed to
implement the painting logic and shouldn't be called directly. A public
repaint method has been introduced to let canvases notify their
associated painters that they need to be redrawn. The default
implementation simply calls paint() immediately; threaded painters,
however, override it to schedule render jobs that run in separate
threads.

All existing threaded painters have been updated to reduce the
visibility of their paint methods and provide public repaint functions.
Consequently, QtThreadedCanvas can now properly handle repaint events by
delegating to its associated painter's repaint method.

Revert "* fixes Issue #99"
This reverts commit 97fb9f3c.