AbstractKickoffItemDelegate: Forcibly ungrab mouse when dragging
Without this patch, mouse input gets stuck until you click somewhere in kickoff.
This solution is kind of ugly since we're putting a DragHandler in a MouseArea, which should already handle drags fine. Unfortunately, I could not determine where the fundamental issue comes from.
I could not improve the code by switching from a mouse area to all pointer handlers. The code for determining the input device type or pointer type for enabling/disabling drag or long press wasn't reliable enough and would double trigger the action unless I accepted all taps first, which in turn prevented other handlers from working correctly. I also could not improve the code by using the base AbstractButton signals and only using pointer handlers for drag and right click for similar reasons.