Sets the default comparison function used when sorting to be sort_func.
If the current sort column id of this is gtk_tree_sortable_default_sort_column_id, then the model will sort using this function.
If sort_func is null, then there will be no default comparison function. This means that
once the model has been sorted, it can’t go back to the default state. In this case, when the current sort column id of
this is gtk_tree_sortable_default_sort_column_id, the model will be
unsorted.
| this |
A `GtkTreeSortable` |
| sort_func |
The comparison function |
| destroy |
Destroy notifier of |
| user_data |
User data to pass to |