Merge remote-tracking branch 'features/properpaint'

Merge remote-tracking branch 'features/properpaint'

Now painting the workarea is done at paint events as should be.
Explicit painting after updating metrics has been replaced by a much
lighter procedure (updatePosCache) to update the insets positions cache.

Expected benefits:
- better performance
- proper use of subpixel aliasing

The LyXRC variable use_qimage is not needed anymore and is therefore removed.