Split/join tokens that were wrongly tokenized

[imnasnainaec]

If we tokenize two separate words as a single token, the user should be able to split them, and that should be remembered when the book is retokenized in the future.

[imnasnainaec]

Two UI ideas:

• For situations with lots of manual tokenization, we may want a separate mode/view
• For one-off token adjustment, there needs to be ways to do that without confusing with the phrase construction