[IChangeLoader] Added a method for retrieving an Identifier.Resolver

This makes it possible for participants to use IChangeDescriptors
[RedXMLUndecorator] Attempt to treat the Resolver as a Bigraph

This is slightly hackish -- I should really either document this behaviour
or replace it with something cleaner
[ExtendedDataUtilities] Added a change descriptor for setting comments

ChangeExtendedDataDescriptor is now far too unsafe -- in particular, it
makes it too easy to bypass the validation and normalisation policies of
extended data properties. This commit begins the process of deprecating it
[ExtendedDataUtilities] Added a base class for extended data descriptors

This new base class is derived from the comment descriptor introduced in
the last commit
[ExtendedDataUtilities] Added a change descriptor for setting aliases
[RedXMLEdits] Added support for alias change descriptors
[EditXMLSaver] Don't bother exporting empty change descriptor groups