Revert "Deprecate Gtk::ActionGroup, *Action, Activatable (sort of), UIManager."

Because we do not believe that the C API is ready to be deprecated.
See https://mail.gnome.org/archives/gtkmm-list/2013-October/msg00003.html

This reverts commit d1cfb5b5.

We should apply this commit again some time in the future.
https://git.gnome.org/browse/gtkmm/commit/?id=d1cfb5b5d805b2033334c8fb8845522e79333814