Extra key signatures appear unexpectedly in parts from multi-staff instruments, and fix itself on save/reload

Thanks as always for the fantastic investigation, it's now easy enough to see what is happening and what needs to be done to fix it. We're still working on a few details, though, because it appears there may be other issues connected to this as well.

For what it's worth, the code in question was supposed to only be relevant when creating parts from voices. But due to one bug that was already present in the code before the change in question, it actually also affected any part made from instruments that have multiple staves. This code really should not have executed at all in that case, so we probably need to fix that too. But this code was also supposed to have been skipped for all those key signatures that are automatically generated on each system, and that's the real bug here. That bug also was already present ever since 3.0, but we never saw it before of the other bug with voices-to-parts that was actually fixed by the change in April - that bug prevented the other bug from being seen.

So really there are three interconnected bugs here:

1) the original bug where key changes were not being copied to parts correctly when using voices-to-parts
2) the newly unconvered bug where extra key signatures get generated in some cases (this will also happen, I predict, when using voices-to-parts)
3) the fact that the code meant to apply to voices-to-parts is also getting executed for parts generated from multi-staff instruments

So whatever fix we come up with we will want to make sure applies to all three

PR courtesy of @njvdberg: https://github.com/musescore/MuseScore/pull/6345

PR had been merged 3,x and master, no idea why that didn't trigger this issue to get marked fixed

Automatically closed -- issue fixed for 2 weeks with no activity.