Cleanup the QDrag when doing a drag with the right mouse button on Mac

When doing a drag with the right mouse button on Mac then it will be
ignored straightaway because it will not be accepted. Therefore we
need to cleanup the QDrag object as normal so that it is deleted as
appropriate.

Change-Id: I781fcea7b7c75775adb814090e917363fa7c9189
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>