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.
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.