[dnd] Optionally restore the drag actor

Currently, the drag and drop code assumes that on a successful drop
the target will either consume the drag actor or that it is otherwise
OK to destroy the actor.
As the drag behavior for window preview was changed, dropping a preview
on the dash now results in the preview being swallowed - to fix, add an
option to restore the actor in case of a successful drop as well.

https://bugzilla.gnome.org/show_bug.cgi?id=619203

js/ui/workspace.js

@@ -125,7 +125,8 @@ WindowClone.prototype = {
                            Lang.bind(this, this._onLeave));
 
         this._draggable = DND.makeDraggable(this.actor,
-                                            { dragActorMaxSize: WINDOW_DND_SIZE,
+                                            { restoreOnSuccess: true,
+                                              dragActorMaxSize: WINDOW_DND_SIZE,
                                               dragActorOpacity: DRAGGING_WINDOW_OPACITY });
         this._draggable.connect('drag-begin', Lang.bind(this, this._onDragBegin));
         this._draggable.connect('drag-end', Lang.bind(this, this._onDragEnd));