Support for checkpoints

[rodmaia]

PR #136 introduces support for checkpoints. While it's a useful feature to simplify story creation, the UX lacks the simplicity the rest of the app has.

For example: checkpoints are automatically named by the app. It's hard to remember the content of a checkpoint based on a generic name. Also, it bundles checkpoints and utters on the same scrolling list. Conceptually they are different things, which can cause confusion. It also elongates the list, making it harder to navigate.

All that being said, which is the best approach to the feature? Leave the PR open and prioritize it on the backlog? Accept the PR but disable the feature? Or something else?

@RochaCarla any thoughts?