Order entries in box based on index, allow theme to override.

include/widgets/widget-internal.h

@@ -24,6 +24,8 @@ struct _widget
     gboolean          enabled;
     /** Expand the widget when packed */
     gboolean          expand;
+    /*** The packing index */
+    int               index;
     /** Place widget at end of parent */
     gboolean          end;
     /** Parent widget */