It is a looong ago promised patch. It fixes some more methods to be const
and add overloaded methods like:
Obj* Foo::bar();
const Obj* Foo::bar() const;
and
Glib::RefPtr<Obj> Foo::bar();
Glib::RefPtr<const Obj> Foo::bar() const;

But I didn't make all possible changes. I'm just not sure what to do with
most methods returning RefPtr<Gdk::...> and RefPtr<Pango::...>.
Also it is not obvious for me if Gtk::Widget, Gtk::Style and other objects'
methods should return const pointers, so I did not modify them.