ActivitiesButton: close the dummy menu when activating the button

We need to close the menu to make sure that any grabs held by
the PopupMenuManager are released, and events are properly delivered.

https://bugzilla.gnome.org/show_bug.cgi?id=707852

js/ui/panel.js

@@ -611,6 +611,7 @@ const ActivitiesButton = new Lang.Class({
 
     _onButtonRelease: function() {
         Main.overview.toggle();
+        this.menu.close();
     },
 
     _onKeyRelease: function(actor, event) {