libgimp: nicer API for functions returning a list.

I.e.: gimp_image_get_(layers|channels|vectors)(), gimp_image_list() and
gimp_item_get_children().
Instead of returning an array of IDs, these will now return a GList with
the right objects ready to use.

libgimp/gimpitemcombobox.h

@@ -58,7 +58,7 @@ GType       gimp_vectors_combo_box_get_type  (void) G_GNUC_CONST;
 #ifndef GIMP_DEPRECATED_REPLACE_NEW_API
 
 typedef gboolean (* GimpItemConstraintFunc) (GimpImage *image,
-                                             gint32     item_id,
+                                             GimpItem  *item,
                                              gpointer   data);
 
 GtkWidget * gimp_drawable_combo_box_new (GimpItemConstraintFunc constraint,