menu: lazily generate menu scenes

menu: lazily generate menu scenes

This removes the need to call update_client_list_combined_menu()
and update_client_send_to_menu() every time a root menu is opened.

This commit also fixed the incorrect menu position with following
configuration:
  <menu id="foo" label="foo">
    <item label="aaaaaa"/>
    <item label="bbbbbb"/>
  </menu>
  <menu id="root-menu">
    <menu id="foo" />
    <menu id="foo" />
  </menu>