title: add keyboard shortcuts to GIo.Menu
body: <p>I am trying to add accelgroup (keyboard shortcuts) to the Gio.Menu items.
  But I have not found a way to do this. Like, in this example, I want to open file with <code>&lt;Ctrl&gt;+o</code></p>
  
  <pre><code>filemenu = Gio.Menu()
  filemenu.append("Open","win.open")
  
  accelgroup=Gtk.AccelGroup()
  self.add_accel_group(accelgroup)
  
  # Open menu
  open_action = Gio.SimpleAction(name="open")
  # this line is problematic
  open_action.add_accelerator("activate". Gdk._keyval_from_name("O"))
  open_action.connect("activate", self.MenuElem.file_open_clicked)
  self.add_action(open_action)
  </code></pre>
  
  <p>How can I do this?</p>