hidpi: Make many places Hi-DPI aware.

This also introduces a getter for the mnDPIScaleFactor variable.

Change-Id: I02ba6858fb1842f911d62976f4c54afc3bfa337f
Reviewed-on: https://gerrit.libreoffice.org/8517
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>