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Explicit painting after updating metrics has been replaced by a much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.

diff --cc lib/configure.py
Simple merge

diff --cc src/LyXRC.cpp
Simple merge

diff --cc src/LyXRC.h
Simple merge

diff --cc src/TextMetrics.cpp
Simple merge

diff --cc src/frontends/qt4/GuiView.cpp
Simple merge

diff --cc src/frontends/qt4/GuiView.h
Simple merge