WidgetPath: Deprecate regions methods.

* gtk/src/widgetpath.hg: Deprecate iter_add_region(),
  iter_remove_region(), iter_clear_regions(), iter_list_regions()
  and iter_has_region() using the rather useless deprecation
  comments from GTK+:
  https://git.gnome.org/browse/gtk+/commit/gtk/gtkwidgetpath.c?id=93b31790424cbc2face0a885b5dd49889c2f297e