panelMenu: Attempt to navigate menus only if we can't navigate inside the menu

This enables key navigation in the calendar.

https://bugzilla.gnome.org/show_bug.cgi?id=667434

js/ui/panelMenu.js

@@ -184,6 +184,9 @@ const Button = new Lang.Class({
     },
 
     _onMenuKeyPress: function(actor, event) {
+        if (global.focus_manager.navigate_from_event(event))
+            return true;
+
         let symbol = event.get_key_symbol();
         if (symbol == Clutter.KEY_Left || symbol == Clutter.KEY_Right) {
             let group = global.focus_manager.get_group(this.actor);