Changes the appearance of the sort indicator.
This does not actually sort the model. Use set_sort_column_id if you want automatic sorting support. This function is primarily for custom sorting behavior, and should be used in conjunction with set_sort_column_id 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; see set_sort_indicator.
| this |
a `GtkTreeViewColumn` |
| order |
sort order that the sort indicator should indicate |