minor checkpoint cleanup

- remember list of checkpoints browser-side
- cleanup references in MenuBar restore list.
  There was a closure issue, where multiple menu items would actually all restore the same checkpoint.
Merge pull request #3939 from minrk/checkpoints

minor checkpoint cleanup

remember list of checkpoints browser-side
don't clobber list when a new checkpoint is created
cleanup references in MenuBar restore list. There was a closure issue, where multiple menu items would actually all restore the same checkpoint.