HiDPI Support for Okular

Summary:
This patch enables HiDPI throughout the application

Every pixmap is multiplied by the devicePixelRatioF
QPainter code is ajusted to take the DPR value into account

All pixmaps get cached with the highest DPR of all screens. When moving the application to another screen, the cache doesn't have to be invalidated.

BUGS: 362856 383589
REVIEW: D6268