GObject has previously allowed installing properties after class_init
has finished running. This means that you could install some of your
own properties on G_TYPE_OBJECT, for example, although they wouldn't
have worked properly.
Prevent this from happening. Require that all properties are installed by
the time class_init has finished.
Complaints go to this bug:
https://bugzilla.gnome.org/show_bug.cgi?id=698614