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 
Since every child view also has a parent options attribute, perhaps you can just check to see if it has a parent.  If it doesn't, it's a top-level element.  That avoids an O(n) search through a view id list.