Sets (or unsets) the function used for sorting items.
If sort_func is null, all items are considered equal.
If the sort func changes its sorting behavior, gtk_sorter_changed needs to be called.
If a previous function was set, its user_destroy will be called now.
| this |
a `GtkCustomSorter` |
| sort_func |
function to sort items |
| user_data |
user data to pass to |
| user_destroy |
destroy notify for |