Widget persistence API

Instead of persisting widgets ourselves, this PR provides the tools to allow users to persist widget states however they want.

This PR also includes some bugfixes, one of which is the removal of views on cell re-execution!!!
Based on #6664 

- [x] Try it
- [x] Include examples

@ellisonbg @SylvainCorlay @jasongrout 
Actually, you could design a Javascript model that used custom messages and custom view creation logic to create a view that isn't displayed in a cell.