- AttributeListener has been refined such that the event

- AttributeListener has been refined such that the event
is fired before the mutation actually occurs.  Addtionally,
the append() and set() methods must now return the given value,
which is used as the value to be used in the mutation operation.
This allows creation of validating AttributeListeners which
raise before the action actually occurs, and which can change
the given value into something else before its used.
A new example "validate_attributes.py" shows one such recipe
for doing this.   AttributeListener helper functions are
also on the way.