Custom / long term serialization support for StateHolder API

[mysticfall]

Need a custom serialization support for StateHolder API to support long term persistence of the model state (i.e. saving as a file for the Pentaho platform).

[mysticfall]

On second thought, I don't think we should use StateHolder API for long term persistence at all. The main usage for the API was to provide short term state management in such a case as session state needs to be serialized.

For the long term persistence, or externalization I don't think it should store model's internal state like ExpNode tree like the StateHolder API does.

Instead, I think the externalization process only relies on the public properties of the model and renderer instance, which can be easily implemented according to each project's needs.