[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/workspacesView.js

@@ -1244,7 +1244,6 @@ SingleView.prototype = {
             this._timeoutId = 0;
         }
         this._dropGroup.lower_bottom();
-        actor.opacity = 255;
         this._inDrag = false;
         this._computeWorkspacePositions();
         this._updateWorkspaceActors(true);