Usage of FlexLayout with external Model control

[brownstein]

I have a project that uses Redux to store and update JSON layouts leveraging the Model.toJson and Model.fromJson functions. In early versions of FlexLayout, components would persist between Model updates, but when I jumped to 9.0.0, I noticed that this was no longer working. For 9.0.0, I shimmed over this issue by manually setting the movableElement of the new TabNodes after re-hyrading from JSON to match the previous model's movableElement (e.g. mounted sub-component).

When I went to upgrade to 9.0.1 to resolve issues related to drag-and-drop, I noticed that this API on TabNode had become private, blocking my upgrade.

I've put together a PR to allow fromJson to hydrate using a previous Model instance at #522 , but I'd like to start a wider discussion about supporting this use case and see if we can come up with an even better solution.