Revision 1426

Use new CLASS_CHANGED message.
(Classification): Inherit from Publisher.
(Classification.__init__): Remove the layer parameter.
        Classifications no longer need to have a parent layer.
(Classification.GetField, Classification.GetFieldType,
        Classification.SetFieldInfo): Removed. The field name is stored
        in the layer, and the type can be retreived by calling
        Layer.GetFieldType().
(Classification._set_layer, Classification.GetLayer): Removed.
        Classifications no longer have a parent layer.