set up a "commit" relations function

[jonorthwash]

Set it up so that relation and token modifications are displayed somehow as draft (e.g., greyed out or as a separate tree), and that some sensible keyboard shortcut (ctl+enter?) "commits" them. I.e., the main tree is updated with those changes only when the changes are committed. This should be an enablable/disablable mode, so people can either edit live or keep the original available as long as they need it.

[jonorthwash]

I think this might actually be several separate but related features:

1. A way to display differences between two graphs (sounds difficult, but is the basis of the rest of this);
2. A mode where differences since last "commit" are displayed;
3. A difference in terminology between an edit (one undo step)—which should also be saved, at least short-term—and a commit (a collection of edits)—which should probably be saved for long-term examination.
   • Default to committing when advanced to next sentence, or at end of session, or ...?
   • Save edit history as [short-term?] browser data, save commit history to backend?

When we decide on what we really want here, we should split this issue into separate issues based on each aspect of functionality.