added support for a "default vtable" per interface, that interface vtables

added support for a "default vtable" per interface, that interface vtables

Tue Sep  2 19:37:21 2003  Tim Janik  <timj@gtk.org>

        * gtype.[hc]: added support for a "default vtable" per interface,
        that interface vtables are initialized from.
        the default vtable is initialized and finalized through class_init,
        class_finalize and class_data from the interfaces GTypeInfo struct.
        (type_data_last_unref_Wm): unload child plugin before unreffing
        parent type.

        testifaceinit.c: minor fixups. fixed up base_init() assertions, since
        with a default vtable, base_init() may be called multiple times.
        added default initializer to iface1.