Add dragging functionality to the labels showing a pixmap
cursor, allowing for testing DnD with pixmaps with device pixel ratio.
Task-number: QTBUG-46068
Task-number: QTBUG-50938
Change-Id: If6781f380864e614efd4328e8b880b57cd900511
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>