use QOpenGLWindow instead of QOpenGLWidget

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...