Composing mulitiple XML files using 1 or more XSLTs

(In [2312]) Fix #1531. Added an entity-loader to handle relative links inside the stylesheet and/or xml-doc. As part of this did some refactoring and created a TransformSource? class and appropriate subclasses to encapsulate and simplify all the code related to the differing sources.

The problem was that if the docs were pulled from something other than a direct file then parsing was on a stream with no url associated with it, and hence there was no way for the parser/transformer to resolve relative urls. Additionally the transformer doesn't know how to load urls with arbitrary schemes. So, we now make sure a url is always associated with each source being parsed and we register an entity-loader that knows how to resolve our internal urls that represent things like repository entries or attachments.