DraggableElement: Make toggling draggability consistent
DraggableElement: Make toggling draggability consistent

* Move some setup code from constructor to #toggleDraggable
* Make the constructor call #toggleDraggable

Bug: T172781
Change-Id: I569bb9b5c072c6928a4379cd5ff64b784d775e12

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