Continue on Implementing GDocument as DomDocument

* This make API changes on GXml.Document, by renaming
  create_element() and create_comment() to create_element_node() and
  create_comment_node(), this is to privilage DOM4 API

* Added missing interfaces and implementations to implement most of
  DOM4 DomDocument

This is a commit to save changes, it doen't compile, requires to fully
implement all interfaces in Element and other objects to DOM4 interfaces