add _gtk_widget_get_aux_info_or_defaults()

add _gtk_widget_get_aux_info_or_defaults()

This is better than peeking aux info then testing != NULL
in several ways:
- it returns const aux info so if we don't create we can't write
- it ensures that the default we assume if aux_info is NULL is
  the same as the default we set if we've created the aux info
- it avoids typing in != NULL checks