Support for DOM and other external tree models

Support for JDOM2 is added.

Support for Apache Axiom is added.

A number of optimizations have been made to the support modules for other external object models, noticeably to speed up navigation of the descendant axis.

A number of helper classes have been introduced to achieve greater commonality between the support modules for different object models. This should hopefully make it easier to add support modules for user-specific data mdodels.

The class DocumentBuilderFactoryImpl, which constructed a DOM wrapper around a TinyTree, and which has been deprecated since 9.3, has now been removed.