Sets the current sort column to be sort_column_id. The sortable will
resort itself to reflect this change, after emitting a
#GtkTreeSortable::sort-column-changed signal. sort_column_id may either be
a regular column id, or one of the following special values:
Sets the current sort column to be sort_column_id. The sortable will resort itself to reflect this change, after emitting a #GtkTreeSortable::sort-column-changed signal. sort_column_id may either be a regular column id, or one of the following special values: