Don't check for Workspaces.WindowClone for window drops

When checking the type of a DND source, instead of checking
'instanceof Workspaces.WindowClone' accept any actor with realWindow
and metaWindow properties. This will be useful to support a separate
type of actor dragged from workspace thumbnails.

https://bugzilla.gnome.org/show_bug.cgi?id=640996

js/ui/workspace.js

@@ -1412,7 +1412,7 @@ Workspace.prototype = {
     },
 
     acceptDrop : function(source, actor, x, y, time) {
-        if (source instanceof WindowClone) {
+        if (source.realWindow) {
             let win = source.realWindow;
             if (this._isMyWindow(win))
                 return false;