New system bar "shirt pocket", for quickly stashing draggable bits.

Anything that can be dragged using the new drag manager APIs
(View#startDrag) can be dropped here; the pocket will hold
it for you until you're ready to drag it out again.

Known issues:
 - currently discarding the clipping whenever you drag it
   out; in the future we will only discard it once you
   actually drag it somewhere
 - there's a bug where dragging things back out of the
   pocket's window requires an extra tap (i.e. tap once then
   tap and drag)
 - almost nothing uses the new drag manager yet :)

Change-Id: Iff3103e4117f31023e1e96667632d77912e21d30