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.

A previous patch asserted that this was not true and we had to revert it
because it broke the shell.  Instead of reverting, we should have used a
critical, so do that now.

Complaints go to this bug:

https://bugzilla.gnome.org/show_bug.cgi?id=698614