* Removed custom libxml-2.0.vapi file

* Added gxml/xlibxml.c/.h and xlibxml.vapi files to implement missing API from
  Vala's libxml-2.0.vapi file (updates to new GXml.Xmlx API)
* Old libxml-DocumentType uses a Xml.Doc to access its internals
* Fixed get_last_error to_string implementation to use string.replace()
* GXml.Node.namespaces changed to Gee.List<Namespace> (updated implementations)
* Added GXml.Node.set_namespace (currently is buggy)
* Added GXml.Serializable.set_namespace and removed serializable_set_namespace
  property (updated implementations)
* Old libxml-Node and BackedNode changed type from NodeList to List<Namespace>
  this change doesn't broke DOM API
* Reimplemented libxml-NamespaceAttrNodeList to access directly to Xml.Node.Ns
  lists of namespaces and implement Gee.List<Namespace> interfaces