A GtkTreeIterCompareFunc should return a negative integer, zero, or a positive integer if @a sorts before @b, @a sorts with @b, or @a sorts after @b respectively.
If two iters compare as equal, their order in the sorted model
is undefined. In order to ensure that the
GtkTreeSortable behaves as
expected, the GtkTreeIterCompareFunc must define a partial order on
the model, i.e. it must be reflexive, antisymmetric and transitive.
For example, if @model is a product catalogue, then a compare function
for the “price” column could be one which returns
price_of(@a) - price_of(@b).
Gtk::TreeModel, Gtk::TreeIter, Gtk::TreeIter -> Int32