Widget: Make insert_action_group() public.
Widget: Make insert_action_group() public.

* gtk/src/widget.hg: Because it is convenient for me in Glom,
  where I need to call it in a helper function rather in the widget
  class itself, to avoid duplication.

johne53/MB3Gtkmm,johne53/MB3Gtkmm,GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm,GNOME/gtkmm,johne53/MB3Gtkmm