Fast scroll letters should use the sort by field

[9vult]

There's two related things here, let me know if I should split the issue. (Also if this has been fixed for the redesign, I'm on stable)

First, the letters currently only consider the "real" title, which unfortunately isn't ideal when there's multiple languages. This means that if there's a letter ('K' in this example) that has no matching "real" titles, but matching "sort by" titles, those are skipped.

Also, if there's no matches, it awkwardly scrolls all the way to the end; it probably would make more sense to scroll to the next existing letter instead (so 'L', or if there's no 'L's, then 'M', etc).

Attaching a screenshot of the library and a video of the scroll in action:

trim.318100B1-77F3-401A-B14E-0C9513B563B5.MOV