[PATCH] add the binding friendly join_group method to GtkRadioAction

[PATCH] add the binding friendly join_group method to GtkRadioAction

* Due to object ownership issues it is impossible to correctly use
  get_group/set_group from a GI binding
* join_group is safer because at the binding level it works with individual
  GtkRadioAction objects and not with the list of objects that gets
  modified in the library