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 
We immediately load the states because this is the same code that executes on refresh.  If no state has been saved yet, the user defined callback should be aware of that and return an empty state.