How to implement a submenu for a GtkAction

[Ma Xiaojun <damage3025 gmail com>]

I'm trying to add a feature to File Roller like this:
http://dl.dropbox.com/u/45139465/The%20Archive%20Browser.png

However, I can only add a single entry to File Roller's File (Archive)
menu. I cannot have a sub menu.
I tried adding a <menuitem> in ui XML, but I cannot have sub menu on it.
I also referenced Gedit's code of HighlightModeMenu, it uses a <menu>
and inside a <placeholder> in it as ui XML. I tried the same thing but
now even the single entry is invisible. I haven't understand and mimic
how Gedit dynamically add submenu entries though.

Thank you in advance.