[gtkmm] libglademm to create an is_a relationship

[Thomas Muldowney <temas box5 net>]

I hope this isn't often asked here, searches of the list didn't turn it
up, so... Has anyone devised a way to use libglademm to create an is_a
relationship from a widget defined in a glade file?  We have a partially
working solution using just regular libglade, but it feels hackish and
incomplete.  Any ones thoughts or comments would be helpful.

--temas