js: Name all the timeouts and idles

With very uninventive names. Names now, good names later.

https://bugzilla.gnome.org/show_bug.cgi?id=727983

js/ui/panel.js

@@ -595,6 +595,7 @@ const ActivitiesButton = new Lang.Class({
             Mainloop.source_remove(this._xdndTimeOut);
         this._xdndTimeOut = Mainloop.timeout_add(BUTTON_DND_ACTIVATION_TIMEOUT,
                                                  Lang.bind(this, this._xdndToggleOverview, actor));
+        GLib.Source.set_name_by_id(this._xdndTimeOut, '[gnome-shell] this._xdndToggleOverview');
 
         return DND.DragMotionResult.CONTINUE;
     },