gtk2-perl: classname functions

Daniel added resolution of Gnome2 and Gnome2::Art 
classnames Gtk2::_Helpers

That is OK in the short run,
but this is not a generalized enough 
because adding new modules are impossible
without changing a core class.

I was thinking this over and we need some kind 
of registration mechanism instead of the hard code
class handlers.
Ideas, objections?

/göran