Extend modifier key support

In designer it is possible to drag widgets around and, e.g., move them
to a different place in a dialog/layout. This is indeed helpful (esp.
the visual feedback with layouts...).

To copy a widget it is necessary to press the Ctrl button first and then copy the widget. It would be helpful if copying could be enabled and disabled during the dragging by holding or releasing the Ctrl key as usual in many other applications.