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 
so if you request state from the javascript side, and the python side, for other reasons, sends an unrelated `set_state` for a single model, the promise will get resolved too soon, right?