demo: Add TreeModel interface implementation demonstration
demo: Add TreeModel interface implementation demonstration

Add demo which shows how to implement the Gtk.TreeModel interfaces virtual
methods.

pexip/pygobject,davibe/pygobject,sfeltman/pygobject,sfeltman/pygobject,davibe/pygobject,sfeltman/pygobject,pexip/pygobject,davibe/pygobject,GNOME/pygobject,davibe/pygobject,GNOME/pygobject,GNOME/pygobject,pexip/pygobject