Make 'Select note/rest above' (and below) work on list selections

Your idea

The shortcuts 'Select note/rest above' and 'Select note/rest below' (Alt+Up and Alt+Down respectively) currently only work on single-note selections, not list selections of multiple notes.

https://github.com/user-attachments/assets/3cb5a539-b404-4eb4-94e6-685dd3bffdd2

Problem to be solved

The tricky part will be handling more irregular list selections, but we could do something like the following:

If you start with this...

One 'Select note/rest above' would get you this...

Running it again would get you this (and running it any number of times more, since the selection is now already at the top of each chord)...

Then, one 'Select note/rest below' gives this...

And one more gives this...

Prior art

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Additional context

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Checklist

• This request follows the guidelines for reporting issues
• I have verified that this feature request has not been logged before, by searching the issue tracker for similar requests

Start by locating the handlers for the “Select note/rest above” and “Select note/rest below” shortcuts, Alt+Up and Alt+Down, and trace how they process single-note versus list selections. Exercise the irregular list-selection cases shown in the issue; done means repeated commands move each selection through the chord toward the top or bottom without breaking the selection.