Use a GType for the PROP_SYMMETRY property of GimpImage, and create
a default "identity" symmetry for an image.
I still use a GimpIntComboBox but store the property value in the
user-data column because gpointer isn't a subset of gint.
Adds in libgimpwidgets:
- gimp_int_combo_box_set_active_by_user_data()
- gimp_int_combo_box_get_active_user_data()
- gimp_int_store_lookup_by_user_data()
- gimp_prop_pointer_combo_box_new() to create a GimpIntComboBox and
  attach it to a gpointer property.
Thanks Massimo and Mitch for reviewing my code.