GtkBox: Move overridden property ids to the end

This will make it easier to convert to g_object_install_properties
in the next commit.

gtk/gtkbox.c

@@ -90,10 +90,12 @@
 
 enum {
   PROP_0,
-  PROP_ORIENTATION,
   PROP_SPACING,
   PROP_HOMOGENEOUS,
-  PROP_BASELINE_POSITION
+  PROP_BASELINE_POSITION,
+
+  /* orientable */
+  PROP_ORIENTATION
 };
 
 enum {