Added Unit Test for override Serializable.transformable_from_string

* Added example to serialize a Gee collection
* Fixed some compilation warnnings
SerializableObjectModel.(de)serialize can be overrided. Added Node.add_namespace_attr()

* Added test case to override serialize, by adding default implementations
  to be called by overriders
* Added default implementations for (de)serialize(_property)
* Added new Node.add_namespace_attr and BackedNode implementation
Fixed bug when using custome node name

* Added test case for custome node name
* Added test case when overriding deserialize
* Added documentation to Serializable.ignored_serializable_properties
Added documentation to Serializable.unknown_serializable_property

* Added test case for unknown property saved to
  Serializable.unknown_serializable_property
Added Node.copy(). More Unit Tests.

* Added Node copy method, Element and Attr implementations too.
* Fixed unknown attributes de/serialize () for SerializableObjectModel
* All unknown properties are serialized back again to original file, to
  avoid data lost on de/serialize operation
* Fixed issue with Element content text, using Text node representation
* Added unit test for unknown properties de/serialize
* Added unit test to avoid serialize properties set to null