Now that both arguments to the _new_with_factory() constructors
are nullable, there's no good reason to keep a separate _new()
around. Just make gtk_list_view_new() and gtk_grid_view_new()
take both a model and a factory.
Change the apis in GtkListView, GtkColumnView and
GtkGridView to be explicitly about GtkSelectionModel,
to make it obvious that the widgets handle selection.
Update all users.
Make both gtk_grid_view_new and gtk_grid_view_new_with_factory
take a model as first argument, and make all arguments
allow-none and transfer full.
Update all callers.
