Mark final (non-derivable / non-subclassable) classes

[selurvedu]

Also known as "sealed" in C#.

References:
https://gitlab.gnome.org/GNOME/vala/-/issues/1036
https://gitlab.gnome.org/GNOME/glib/-/issues/2321
https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1937
https://gitlab.gnome.org/GNOME/gobject-introspection/-/merge_requests/257
https://gitlab.gnome.org/GNOME/gobject-introspection/-/issues/368#note_978855

The last link contains some explanations about how to determine if a class is final when it does not explicitly have a corresponding type flag. The flag was introduced quite recently in GLib 2.70 and gobject-introspection 1.70, but a convention of non-derivable classes has been around for years.

gi-docgen already supports those, hence docs.gtk.org does as well.