Improved TwDocument Namespace handling

* By default no prefix are added for default namespace
* Added GXml.Document.prefix_default_ns to add default namespace's
  prefix
* By setting a namespace to GXml.Document/TwDocument, first one
  is used as default for all document
* Added GXml.Document.ns_top to add all namespaces declarations to
  root node (still fails to remove its declaration in children nodes)