Changes the appearance of the sort indicator.
This does not actually sort the model. Use set_sort_column_id() if you
wnat automatic sorting support. This function is primarily for custom
sorting behavior, and should be used in conjunction with
GTK2.TreeSortable->set_sort_column() to do that. For custom models, the
mechanism will vary.
The sort indicator changes direction to indicate normal sort or reverse
sort. Note that you must have the sort indicator enabled to see anything
when calling this function.
One of SORT_ASCENDING and SORT_DESCENDING .