Revision 1338

Fixes RTbug #1971, 1973.
(Layer.Destroy): We don't need to call SetClassification
        anymore to clear out the back reference in the classifcation
        to the layer. It's better to set it to None using _set_layer,
        and we won't be creating another clas object too.
(Layer.SetClassification): Classification._set_layer is not
        recursive so remove all the locking variables. Also clean
        up the code so that it remains unchanged if something fails.