workspace: Also hide attached dialogs in WindowClone from picking

As explained in the comment in _init() of WindowClone, we hide the
actual clone from picking so it doesn't interfere with XDND.

This description applies to the clones of the attached dialogs just as
well though, so hide the clones of attached dialogs from picking, too.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1293

js/ui/workspace.js

@@ -239,6 +239,9 @@ var WindowClone = GObject.registerClass({
 
             this._onMetaWindowSizeChanged();
         });
+
+        Shell.util_set_hidden_from_pick(clone, true);
+
         this.add_child(clone);
     }