Besides the device grab on the drag device, also set up a captured-event
handler to catch other devices (except the keyboard) while the DnD
operation is ongoing. This makes DnD operations exclusive to others.
Also, disallow it in less aggressive ways if maybeStartDrag() gets called
while there is a current draggable.
This might definitely be nicer (eg. having other grabbed devices emit
leave/end events), but can't be done without major surgery to Clutter.
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