gtype: disallow adding interfaces after the fact

gtype: disallow adding interfaces after the fact

Add a check to prevent adding an interface to a class that has already
had its class_init done.

This is an incompatible change but it is suspected that there are not
many users of this functionality.  Two known exceptions are pygobject
(fixed in bug 686149) and our own testsuite (affected tests have been
temporarily disabled by this patch).

Once we confirm that nobody else is using this functionality we can
remove a rather large amount of code for dealing with this case.

https://bugzilla.gnome.org/show_bug.cgi?id=687659