SerializableObjectModel: fixed use_nick property.

* SerializableObjectModel.serializable_property_use_nick = false
  by default
* Fixed when property use nick as GXml.Element property name
* Added test case for property using nick
Added new Serializable.transform_(to/from)_string

* Implemented on SerializableJson/ObjectModel returning false
  and as virtual to allow overriding
Added unit test for override Serializable.transform_to_string()