Make selectXpath simpler to use by disabling namespaces

[jhy]

Current, Element#selectXpath() has namespace awareness for its queries. However, this makes selecting anything from a document with an xmlns attribute set a pain, and expressions end up looking like: //*[local-name()="input"]. See #1746 for an example of the problems this causes.

It will be an improvement to disable namespaces when used in the Element#selectXpath() method. If users really want to use XPath with namespaces they can still use the W3CDom factory, which then also gives the path to registering namespace URI handlers.

[jhy]

Completed by 2b573de