QT5 shows a widget twice when you set a parent at initialization
and reset the parent later. This is exactly what happens when a
widget is initialized and passed to AddWidget in the gosty app.
The solution is to initialize them without a parent and let
the AddWidget function taking care of it.

commit 1268b4b4 just shifted the
problem. DNG nicely shut down but a unit test segfaulted. I did not
investigate in depth what exactly happened. However, the cleanup
at Python shutdown seems to be necessary. The problem is that the
scene destructor might have already been called when the cleanup at
Python exit happens and the pointers to the scene observers point into
Nirvana. So by explicitely defining the Scene destructor that kills
all observer pointers and bound gfx objects, the likelihood for bad things
happening when the RemoveAll function is called on the Singleton again
is reduced.

iCCP: profile 'Photoshop ICC profile': 'RGB ': RGB color space not permitted on grayscale PNG

This manual cleanup led to a segmentation fault on my system, as the
static scene instance was already deleted when this function was called.
At least on my machine I get a clean exit now...

- Directly Register GLCanvas to Scene at initialization
- Stereo Rendering (at least on our setup) fails when setting the alpha buffer
  -> disabled for stereo

Accept compiler warning about unused functions, as they're in the
header file and might be included by other code that requires the
functions to be there. Happened for the unit tests.

Any compilation unit having included QT OpenGL stuff should not get in
touch with anything that has classic OpenGL headers included...

Some people might like to have a raw OpenGLWindow to directly interact
with gfx::Scene

Reason for that is that the latter does not support stereo rendering.

Two features have been removed in the process:

 - CopyToClipboard: copy the current selection on the screen to the
                    clipboard. In the current implementation, this
                    was broken anyway and did nothing.
 - QGesture Support: Requires further investigation whether we need that
                     anywhere. Maybe DTB...

Copied change from PM3 (commit d16162baffb140de971e24aad71c8cce257a6fce).