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

gobject/gtype.c

@@ -4646,3 +4646,13 @@ g_type_ensure (GType type)
   if (G_UNLIKELY (type == (GType)-1))
     g_error ("can't happen");
 }
+
+gboolean
+g_type_is_in_init (GType type)
+{
+  TypeNode *node;
+
+  node = lookup_type_node_I (type);
+
+  return node->data->class.init_state != INITIALIZED;
+}