Added option to sort books in library by oldest unread chapter (next unread chapter).

[meee-low]

This closes feature request #1443, posted by me a few days ago. This aims to make it easier for users to read chapters across multiple series in release order.

This involves a few additional functions for the SQL queries and the new option in the UI.

Tests

I tested that the ascending/descending order matches the already existing "last updated" order, in the case where those are the same (when the only unread chapter is the latest chapter). For titles with no unread chapters, the behavior is to default to alphabetical, like other similar sorting options.

Possible alternative

This implementation interprets "next" as "oldest unread chapter". If a user skipped chapters, that's not quite the case. However, this interpretation matches the current behavior for clicking the open book button in the library (opens to the oldest unread chapter).
An alternative approach would be to find the actual latest chapter read in the user history, then find the next chapter from there.

[meee-low]

Hey, it's been some time. Is the project open for outside contributions like this? Is there something I can/should do to get this reviewed? I'm open for any questions.

Thank you.