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 
Why are we immediately loading widget states?  The very first time a person sets up the callbacks, the states have likely not been saved yet.  I think we probably shouldn't automatically load the widget state.