Implemented the DOML3 XPath interfaces, reusing the XPath processor used inside the XMLSchema validation; this means it understands only a limited subset of the XPath syntax, and can only return DOMElement nodes. However, it could be useful for simple navigations, or maybe be later expanded

git-svn-id: https://svn.apache.org/repos/asf/xerces/c/trunk@502666 13f79535-47bb-0310-9956-ffa450edef68