lazy initialization of container model breaks metadata context

[justin-hackin]

you can't nest tweakable properties inside a container model if you don't create a defaultFn that instantiates it on the property where it is stored. Even though the applySnapshot on open widget preserves the instances' context, it can be useful to have models with indeterminate existence like the stroke dash path properties on the Pyramid Net model.

Potential solution, somewhat inelegant: store metadata into a model registry and use the model name and valuePath as a key for the metadata value. We can only get the valuePath after attachment to root store so the context is still useful but the registry would take precedence for metadata getter with context as fallback. Not sure...