app: don't use g_free() as key_destroy_func in a GType-keyed hash table

This would have crashed if we ever replaced or removed something in
that hash table. Instead, use g_object_unref() as value_destroy_func
because the table keeps GimpContainers which we would leak if we ever
removed anything from the table.

app/gegl/gimp-gegl-config-proxy.c

@@ -267,8 +267,8 @@ gimp_gegl_get_config_container (GType config_type)
   if (! config_containers)
     config_containers = g_hash_table_new_full (g_direct_hash,
                                                g_direct_equal,
-                                               (GDestroyNotify) g_free,
-                                               NULL);
+                                               NULL,
+                                               (GDestroyNotify) g_object_unref);
 
   container = g_hash_table_lookup (config_containers, (gpointer) config_type);