This patch removes qscreen.[Ch] and simplify the cursor drawing on screen. Basically...

This patch removes qscreen.[Ch] and simplify the cursor drawing on screen. Basically, we paint now only if needed. So there's no need anymore to handle the "no cursor" pixmap saving.

SConscript: removed qscreen.C
frontends/qt4/Makefile.am: removed qscreen.C
frontends/screen.h: remove workarea() constness
frontends/qt3/qscreen.[Ch]: ditto
frontends/gtk/GScreen.[Ch]: ditto
frontends/xforms/xscreen.[Ch]: ditto
frontends/qt4/LyXScreenFactory.C: now return the QWorkarea directly
frontends/qt4/QWorkArea.[Ch]: inherits LyXScreen, handle the cursor painting
frontends/qt4/qscreen.[Ch]: removed

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@13981 a592a061-630c-0410-9148-cb99ea01b6c8