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

closes #546
closes #37

* Implemented trackball navigation interaction for rendered geometries
* Implemented correct rendering of textures with correct aspect ration (no longer weirdly stretched textures)
* removed obsolete (because redundant) renderFullscreen option

refs #546
refs #37

* DataContainerInspectorCanvas uses the GeometryRenderer processor to render geometry
* DataContainerInspectorCanvas::_textures now uses DataHandle to improve thread safety avoid segfaults
* replaced some function calls with Qt signals to loose coupling
* removed obsolete fields and methods

refs #546

Removed all code fragments in core code that was ITK specific (conversion from/to GenericImageRepresentationItk). The functionality was replaced by the new conversion functors in imagerepresentationconversionitk.h and imagerepresentationconversionitk.cpp.

refs #553
refs #474

"MERGED 4 commits - 1. cleaned up googletest 2. solved std_tuple issue with four modified files from current beta googletest 3. MTd-MdT issue resolved with CMake flag 4. googletest produces two projects named gtest and gtest-main. a third project is created linked with gtest-main and all our testing libraries"

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

* Introducing base module with LightSourceProvider processor
* Updated GeometryRenderer and GeometryRendererDemo to use LightSourceData instead of ProcessorDecoratorShading
* Introducing LightSourceData::bind()

refs #547

To now, each property hat an _invalidationLevel field that was evaluated by processors when the property had changed in order to determine what has to be done. However, since properties could also be owned by other classes, this design was semantically misleading.
Therefore, it was removed with this commit and replaced by the invalidation map of each processor. Now, this per-processor mapping of property -> invalidation level is managed by the processor itself. Furthermore, the invalidation level is no longer setup during property creation but during AbstractProcessor::addProperty(), which also makes much more sense.

ATTENTION: Due to these intrusive API changes, the code of all processors and other classes handling properties needs to be changed. As a reminder, the implementation of addProperty() also now takes a reference instead of a pointer, so that old code does no longer compile.

refs #542

Module file parsing now supports new fields "ThisModApplicationSources", "ThisModApplicationHeaders", and "ThisModApplicationToBeMocced". These files will be added to the campvis-application package (instead of to the campvis-modules package) and thus for instance allow a module to declare property widgets in a non-intrusive way.

Little bug fix in glvesselnessfilter.frag shader.

* 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.

Closes #387: LogViewerWidget now has a checkbox to select the minimum log level to display. Filtering is performed in widget. Checkboxes to enable/disable each log level, however, seemed to be overkill. If one really wants such specific filtering, he/she can use the filter text box.

Extended the DataContainerTreeWidget and its corresponding DataContainerTreeModel to support a second level containing the DataHandles of RenderData and ImageSeries items.

* Fixed bug introduced in 8a8f9bba: Mouse wheel handler was modifying wrong slices
* Updated README.md

Beautifying VolumeExplorerDemo and VolumeRendererDemo sample rederings by fine tuning transfer functions.

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.

Affected classes:
- Columbia1
- AdvancedUsFusion
- StrainFiberRenderer
- SimilarityMeasure
- GeometryRenderer
- DataContainerFileLoaderWidget
- DataContainerInspectorCanva

* Added assertion in ImageRepresentationGL constructor checking for matching number of channels

* Rearranged DataContainerInspectorWidget layout to use less space
* Added option to render custom channels in DataContainerInspectorCanvas

* 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

GUI beautification: DataContainerInspectorWidget; Save and load next to each other, hide save if no devil module, disable save btn if no item selected

* Fixed SliceExtractor not rendering Y slice position correctly in XZ-plane mode
* Minor improvements in DataContainerInspector fragment shader and GlGradientVolumeGenerator