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 
On the other hand, that means that any child can pretend they are a top-level element.  You're probably right to only rely on "trusted" information like the cell's record of top-level views.