Trivial patch to mark GResolver as abstract. This breaks API, but arguably nobody could have been using the API in the way this breaks, so there should be no impact. I would be happy for the patch to be rejected for breaking API though.

The motivation is to prevent people accidentally shooting themselves in the foot by using (e.g., in Python);
   resolver = Gio.Resolver()
rather than
   resolver = Gio.Resolver.get_default()

It’s a lot harder to get this wrong in C, since g_resolver_new() doesn’t exist. You’d have to do:
   resolver = g_object_new (G_TYPE_RESOLVER, NULL);
rather than
   resolver = g_resolver_get_default ();