DraggableGroupElement: Cache directionality
DraggableGroupElement: Cache directionality

Computed CSS is slow, so just do it once when the drag
starts as it shouldn't change while the user is re-ordering
items.

Change-Id: I4eb1e75b22a79cd593bad38066c34b33e1dda76b

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui