[pygtk] Set "focusable" state

[cwr at netcom.co.uk]

I've been trying to use Accerciser, the Gnome ATK test tool
(I actually want to use Dogtail, but that has problems I won't
go into here).

Accerciser reports on various missing requirements, among them
the need to set "focusable" state on some notebook tabs which
have the "focused" state.  Does anyone know how I can do this
in PyGTK?  I can't find any way of adding state to an object;
atk.Object.ref_state_set() gets a reference to the objects state
set, and not the state set itself.

Thanks - Will