JAXP interface (transformation, XPath, schema processing)

I have removed the restriction that a URIResolver for the URI contained in the href pseudo-attribute of the <?xml-stylesheet?> processing instruction must return a SAXSource. It can now return any kind of Source. This change has been regression tested only. The (existing) code for creating a composite stylesheet when there are several <?xml-stylesheet?> processing instructions, (as specified in the JAXP interface definition) is not tested by any of my standard tests, but I have left it in. From an inspection of the code, I don't think it will work if the URIs are relative.