GMenuModel -> GtkMenu binding: show submenu items

In the non-submenu case we bind the 'visibility' attribute to the
tracker, which takes care of showing the item.  In the submenu case, we
don't bind all of the properties, so we miss this one.

Deal with it by just show()ing the submenu item.

https://bugzilla.gnome.org/show_bug.cgi?id=702332

gtk/gtkmenushell.c

@@ -2132,6 +2132,8 @@ gtk_menu_shell_tracker_insert_func (GtkMenuTrackerItem *item,
           g_signal_connect (submenu, "show", G_CALLBACK (gtk_menu_shell_submenu_shown), item);
           g_signal_connect (submenu, "hide", G_CALLBACK (gtk_menu_shell_submenu_hidden), item);
         }
+
+      gtk_widget_show (widget);
     }
   else
     {