Support all QDateTime string formatting options as filename placeholders

There are a lot of them and showing them all in the UI might be too
much, so they're currently hidden features. I know adding hidden
features is kind of dubious, but I don't think it'll be harmful. We
can't add new translatable strings right now without asking for a string
freeze exception anyway.

See QDate and QTime docs:

- https://doc.qt.io/qt-6/qdate.html#toString
- https://doc.qt.io/qt-6/qtime.html#toString

BUG: 477215