[gtkmm] Gtk::TreeModel derivation

[Benjamin Dauvergne <bdauvergne tol-eressea org>]

I'm currently trying to derive a new base class from Gtk::TreeModel that
would be like the NSTableViewDataSource of Cocoa:
http://developer.apple.com/documentation/Cocoa/Reference/ApplicationKit/ObjC_classic/Protocols/NSTableDataSource.html#//apple_ref/doc/uid/20000122/BCIDGGHE

But i don't see why the get_value_vfunc and get_value_impl are for? They
seem to do the same thing.

I think that a lot could be done to ease the implementation of
TreeModel's with conveniences classes.

@+