GObject: prevent installing properties after init

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