Bug Fixes. API simplification for Serializable Containers.

* Simplified implementing GObjects having Serializable Gee
  collections by adding SerializableContainer interface
  and implementing se/deserialization in SerializableObjectModel
* Serializable Gee Collections now return default node_name
  from its elements
* Fixed bugs on elements' content and unknown properties on
  complex tree with sub-containers
* Fixed bug on unknown Text elements
* Fixes, new ones and improves to Unit Test