dateMenu: Set label_actor for WorldClock button

Ideally we would allow navigating into the button in the screen
reader case, so the configured clocks are read out properly.
However we can still do better than nothing short-term by pointing
to the section header as the button's label_actor.

https://bugzilla.gnome.org/show_bug.cgi?id=745393

js/ui/dateMenu.js

@@ -163,6 +163,7 @@ const WorldClocksSection = new Lang.Class({
                                     x_align: Clutter.ActorAlign.START,
                                     text: title });
         layout.attach(header, 0, 0, 2, 1);
+        this.actor.label_actor = header;
 
         for (let i = 0; i < this._locations.length; i++) {
             let l = this._locations[i].location;