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

* gtk/gtkmm/radiobuttongroup.[h|cc]: Deprecate the add() that takes a RadioAction.
* gtk/src/action.hg: Deprecate Action.
* gtk/src/actiongroup.hg: Deprecate ActionGroup.
* gtk/src/activatable.hg: Add a comment that interface Activatable is
deprecated. Can't do more than that without breaking ABI. It's a base class
of several classes.
* gtk/src/radioaction.hg: Deprecate RadioAction.
* gtk/src/recentaction.hg: Deprecate RecentAction.
* gtk/src/toggleaction.hg: Deprecate ToggleAction.
* gtk/src/uimanager.hg: Deprecate UIManager.

This together with the previous 4 commits should fix the build with
--enable-warnings=fatal until more API is deprecated in gtk+.
These commits don't fix 'make check'. The demos use deprecated API.