Fixed bug on GXml.Namespaces implementation in old libxml

* NamespaceAttrNodeList now is fully ported to Gee.List<Namespace>
* NamespaceAttrNodeList uses xmlGetNsList to find all namespaces the
  document
* Now set_namespace work properly
* Deserialize with namespaces is working
* Added Xmlx.doc_get_ns_list wrapping xmlGetNsList