Adding DraggableGroupElement and DraggableElement mixins

Adding a GroupDraggableElement and DraggableElement to ooui for a
drag/drop usability. GroupDraggableElement is a group element that
contains DraggableElements and responds to drag/drop events by displaying
a poper placeholder and rearranging the items when needed.

Demo added to the demos page.

Change-Id: I859ff276ea97628fde28327d200ed059d018c178