GXml.Serializable API changes. GXml.Node.copy fixes.

* Serializable.unknown_serializable_property renamed and type
  changed to Serializable.unknown_serializable_properties as Gee.Map

* Added Serializable.unknown_serializable_nodes as Gee.Collection

* All Serializable derived classes updated

* Unknown attributes and nodes, are should be stored in a per-object
  TwDocument.root

* Fixed long standing bug about Element.content, by always referencing
  GXml.Text to "built" it and store

* SerializableObjectModel implement above Serializable requirements. All
  other classes just checked not to be affected by the change.

* SerializableObjectModel stores all node's texts as GXml.Text nodes
  and added to childs.

* GXml.Element requires all implementations should use GXml.Text internally
  for texts in node, but concatenate them when get its content

* GXml.Node.copy() fixed Element content copy

* TwDocument API change. Added TwDocument(), TwDocument.for_path() constructors.
  first one creates a tmp file by default.

* Added TwDocument.save_to() to allows save a document to a different file
  without change internal file used by save()

* First implementation of TwDocument.to_string(), not tested yet

* Fixed GXml.Text.value

* xElement.content, follows GXml.Element implemetation, using GXml.Text
  to store its content

* Fixed Unit Test: /gxml/serializable/object_model/deserialize_unknown_property