Introduce an OnDragListener mechanism

If the listener's onDrag(view, event) returns 'false', the view's own
onDragEvent() will be called.  If the listener returns 'true', it
consumes the event (and declares that it will handle the rest of the
drag event sequence, in the case of DRAG_STARTED actions).

Change-Id: I01aff0e4f59f71e55f5eea1049905c80714f0607