sort.column.id. The sortable will
resort itself to reflect this change, after emitting a
gtkTreeSortableSortColumnChanged signal. sortable may either be
a regular column id, or one of the following special values:
GTK_TREE_SORTABLE_DEFAULT_SORT_COLUMN_IDGTK_TREE_SORTABLE_UNSORTED_SORT_COLUMN_IDgtkTreeSortableSetSortColumnId(object, sort.column.id, order)objectsort.column.idorder