The native painting code in the kde5 backend is Qt based only.
To prevent multiple inheritance, it's moved into an extra class
and just leaves the backend specific QImage blitting in the
specific SalGraphics implementation.

Change-Id: I3d5f58f42a37966794541fe1214c1b9557376a98
Reviewed-on: https://gerrit.libreoffice.org/58652
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>