gpointer gis_viewer_add(GisViewer *self, GisObject *object,
gint level, gboolean sort);
void gis_viewer_remove(GisViewer *self, gpointer ref);
gpointer gis_viewer_add(GisViewer *self, GisObject *object,
gint level, gboolean sort);
void gis_viewer_remove(GisViewer *self, gpointer ref);