workspace: Return results of forwarded acceptDrop

Return the results of calls to acceptDrop that we forwarded to the
Workspace object.

This fixes a bug where app icons that were dragged and released above a
window clone would get animated back to their original position
(indicating that nothing happened) even though they opened correctly.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/121

js/ui/workspace.js

@@ -421,7 +421,7 @@ var WindowClone = GObject.registerClass({
     }
 
     acceptDrop(source, actor, x, y, time) {
-        this._workspace.acceptDrop(source, actor, x, y, time);
+        return this._workspace.acceptDrop(source, actor, x, y, time);
     }
 
     _onDragCancelled(_draggable, _time) {