[gtkmm] Yet another custom cell renderer question :-(

[SPAM <SPAM bernard-hugueney org>]

Hello !
I've been searching the list & reading docs & examples.
I've found exemples on how to do a custom celle renderer "from 
scratch" with the custom toggle cellrenderer.
I've also found how to fit a SpinButton.

However, I' have not found how to do what I want :-(

I want to reuse a custom widget that is not a Gtk::CellEditable, so I 
believe I cannot do it "the Spinbutton way".
What is frustrating is that my custom widget is a Gtk::VBox which 
*contains* a Gtk::SpinButton, but with other stuff around.

I dunno if I can inherit from Gtk::VBox *and* Gtk::CellEditable, 
calling my inner SpinButton methods to implement the CellEditable 
methods. First test of multiple inheritance of widgets went wrong :-(

Any help greatly apreciated !

Bernard