`GtkSectionModel` is an interface that adds support for sections to list models.
A `GtkSectionModel` groups successive items into so-called sections. List widgets like `GtkListView` and `GtkGridView` then allow
displaying section headers for these sections by installing a header factory.
Many GTK list models support sections inherently, or they pass through the sections of a model they are wrapping.
When the section groupings of a model change, the model will emit the [signal@Gtk.SectionModel:GtkSectionModel:sections-changed
] signal by calling the [method@Gtk.SectionModel.sections_changed] function. All sections in the given range then need to be
queried again. The [signal@Gio.ListModel:GtkSectionModel:items-changed] signal has the same effect, all sections in that
range are invalidated, too.