Gio::ActionGroup: Add templated getters.
Gio::ActionGroup: Add templated getters.

* gio/src/actiongroup.hg: Add templated get_action_state()
  and get_action_state_hint() methods, adding
  *_variant() methods that they call, and deprecating the
  existing methods that have no output parameters.
  Thanks to Kjell for the suggestion.

johne53/MB3Glibmm,GNOME/glibmm,GNOME/glibmm,johne53/MB3Glibmm,johne53/MB3Glibmm,johne53/MB3Glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm