[DND] Grab keyboard during dragging and abort drag on Escape

This grabs the keyboard focus while dragging an item and aborts the current drag if a press of Escape is detected. It is only partially successful but it doesn't seem to have any negative impact where it fails to grab the keyboard. It works fine in Expo. When dragging from the menu applet, it does not abort the current drag on Escape, but it has the beneficial effect of not closing the menu while the drag is still in progress, if Escape is pressed.