Backport PR
#3939: minor checkpoint cleanup...
Backport PR
#3939: 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.
Issues revealed by rgbkrk/bookstore, which supports multiple checkpoints.
I'm fine if this doesn't get into 1.0, given timing.
