[gtkmm] lifetime fix in gtkmm2 in cvs

[murrayc t-online de (Murray Cumming)]

I've just made a change to the widget lifetime code in gtkmm2 in cvs. We
think that this removes some rare bugs and actually simplifies the
internals code. But I would like people to try it out because there is a
chance that this introduces new
- premature deletions
- memory leaks
- GtkWidget-Gtk::Widget disconnections
and we need to know about that sort of thing.

-- 
Murray Cumming
murray usa net
www.murrayc.com