No order, all elements are considered equal.
gtk_sorter_compare() will only return Gtk.Ordering.EQUAL.
A partial order. Any Gtk.Ordering is possible.
A total order. gtk_sorter_compare() will only
return Gtk.Ordering.EQUAL if an item is compared with itself. Two
different items will never cause this value to be returned.
Describes the type of order that a Gtk.Sorter may produce.