workspace: Fix activating windows from xdnd operations

Commit e5bc3a2ba8 changed the hierarchy of WindowClone, which
broke activating windows on hover during xdnd operations. To
avoid intrusive changes, just hide the new actor from picks so
that DND operations pick the actor actually meant to represent
the corresponding window.

https://bugzilla.gnome.org/show_bug.cgi?id=658640

js/ui/workspace.js

@@ -102,6 +102,12 @@ WindowClone.prototype = {
         this._windowClone = new Clutter.Clone({ source: realWindow.get_texture(),
                                                 x: -borderX,
                                                 y: -borderY });
+        // We expect this.actor to be used for all interaction rather than
+        // this._windowClone; as the former is reactive and the latter
+        // is not, this just works for most cases. However, for DND all
+        // actors are picked, so DND operations would operate on the clone.
+        // To avoid this, we hide it from pick.
+        Shell.util_set_hidden_from_pick(this._windowClone, true);
 
         this.origX = realWindow.x + borderX;
         this.origY = realWindow.y + borderY;