为什么Accelerator Key没有显示在Gtk.Application菜单上？

Question

以下python Gtk应用程序代码在显示一个简单菜单方面非常有效。可以使用快捷键（aka accelerator / accel）激活该菜单。但是钥匙并没有显示在菜单旁边。是什么引起了这个？我在Ubuntu 12.04 64位上使用Gtk 3.4，Python 2.7进行了测试。

from gi.repository import Gio, Gtk
import sys

menus_bar_xml ="""
<?xml version="1.0"?>
<interface>
  <menu id="menubar">
    <submenu>
      <attribute name="label">Menu</attribute>
      <section>
        <item>
              <attribute name="label">Gtk</attribute>
              <attribute name="action">app.version</attribute>
              <attribute name="accel">v</attribute>
            </item>
          </section>
        </submenu>
      </menu>
    </interface>
"""

class Application(Gtk.Application):
    def __init__(self):
        Gtk.Application.__init__(self, application_id="bk2suz.motion_picture")
        self.connect("startup", self.on_startup)
        self.connect("activate", self.on_activate)

    def on_startup(self, app):
        builder = Gtk.Builder()
        builder.add_from_string(menus_bar_xml)
        self.set_menubar(builder.get_object('menubar'))

        action = Gio.SimpleAction.new("version", None)
        action.connect('activate', self.version_callback)
        self.add_action(action)

    def on_activate(self, app):
        win = Gtk.ApplicationWindow()
        win.set_default_size(640, 480)
        win.set_title('Sample')
        self.add_window(win)
        win.show_all()

    def version_callback(self, action, parameter):
         print('Gtk Version {0}.{1}'.format(Gtk.get_major_version(), Gtk.get_minor_version()))


if __name__ == '__main__':
    app = Application()
    app.run(sys.argv)