PopupMenu: handle submenus inline

Instead of showing submenus on the left side, make PopupSubMenuMenuItem
act like an expander. The sub menu is toggled on click, opened on
right/enter/space on the parent item, closed on left on any item
or when closing the parent menu.

https://bugzilla.gnome.org/show_bug.cgi?id=633476

js/ui/panelMenu.js

@@ -50,7 +50,7 @@ Button.prototype = {
         if (open) {
             this.actor.add_style_pseudo_class('pressed');
             let focus = global.stage.get_key_focus();
-            if (!focus || (focus != this.actor && !menu.contains(focus)))
+            if (!focus || (focus != this.actor && !menu.actor.contains(focus)))
                 this.actor.grab_key_focus();
         } else
             this.actor.remove_style_pseudo_class('pressed');