Refactor peek, Toc and Dates

[upsiflu]

The algorithms for

• displaying small previews ("peek")
• showing the date range for a given Segment

were implemented in a lacking way and don't cover the use cases. I'm now refactoring these. This will solve the following issues:

1 #2 #3

I've designed the correct algorithms, but to implement them, module boundaries need to shift (this is called "refactoring").

Implementation will take a day.

severity: severe

[upsiflu]

fixed with minor regressions: 3ca41eaee41bac22cacbcc5b9f41793a9735c38a