Possibility to change custom Drag&Drop cursors while dragging something. Implementati...

Possibility to change custom Drag&Drop cursors while dragging something. Implementation for Windows and X11.

Additional checks to figure out if new Drag&Drop cursors where set. This means it is possible now to keep QDrag object in your program and call setDragCursor() method every time we need to change cursor depending on context.

Change-Id: I4be69e44b2863371a7ffbb29efc17c18210d6cde
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Gatis Paeglis <gatis.paeglis@digia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>