Sliding the workspaces back in when the user starts dragging an item in the
expanded display mode allows the user to select a workspace in which the item
should be launched and stay in the overlay mode.
This patch adds code to dnd.js that handles notifying actors when a drag item is being dragged over them. Overlay code uses such notification to unset expanded display modes and trigger sliding in of the workspaces.
If the drag is cancelled the drag item snaps back to its source or disappears at the original position of its source if the source is no longer displayed.
