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.

DoubleAdjusterWidget displays and provides two ways of modifying
double-precision floating-point numbers: a slider that can be used to
quickly change numeric values, and a spin box, which is better suited
for precise adjustments.

DoubleAdjusterWidget is designed to be used by FloatPropertyWidget and
widgets for vector properties storing floats.

The sizeHint method has been overridden in PipelineTreeWidget to return
a sensible height for the wigdet, while still reusing QTreeView's
hardcoded width.

The main window makes use of that to limit PipelineTreeWidget's maximum
height and give any vertical space reclaimed in this way over to the
widget displaying pipeline properties.

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.

To stop the widget that contains pipeline properties from constant
resizing and taking space away from other widgets, its contents were put
in a scroll area that only resizes horizontally, and scrolls vertically.

Things like date-stamps, time-stamps, categories, and log levels are now
highlighted in every log message. This should make the contents of the
log viewer easier to scan by indicating which information is important.

VisualizationPipelineWidget and PipelineMdiSubwindow have been extended
to report changes in their positions to VisualizationPipelineWrapper.
Based on the information the latter receives, it decides when to undock
MDI subwindows and dock floating pipeline widgets.

As a result, it's now possible to drag visualization pipeline widgets
back into the MDI area and turn them into MDI subwindows.

The way visualization pipelines are displayed has been considerably
reworked. A new class, VisualizationPipelineWrapper, is now responsible
for creating all necessary representations (VisualizationPipelineWidget,
PipelineMdiSubwindow) of a visualization pipeline and seamlessly
switching between them in response to the user's actions (window
dragging, key presses, etc). As a result, it's now possible to drag
visualization pipeline widgets out of the MDI area and turn them into
top-level floating windows.

This initial .gitignore masks build by-products and project files of some
IDEs.

 * each VisualizationProcessor manages its own FBO
 * instead of creating a whole new FBO each process(), the processors shall only create and attach textures to the FBO
 * the FramebufferActivationGuard offers automatic (de)activation and detachment of all textures

SimpleRaycaster already uses the new concept, the rest still uses the legacy API

This reverts commit 240d7dab.

 * added debug output to DataContainerInspector