Fix key signatures regarding concert pitch when cloning staff

[cbjeukendrup]

when either the full score is in concert pitch and the part not, or the part is in concert pitch and the score not.

Resolves: #16554

[cbjeukendrup]

The third commit is an attempt to fix an assertion failure that happened sometimes but I still haven't found a way to reproduce it reliably. It must be some permutation of toggling concert pitch in the full score and in the part score, adding instruments and saving/closing/reopening, and adding key signatures.

The fourth commit fixes an additional bug that still occurred after the second commit:

1. Create score with transposing instrument and concert C major key sig
2. Turn on concert pitch
3. Save close reopen
4. Turn off concert pitch
5. Create new empty part
6. Make the instrument visible in that part
   -> the instrument has the wrong key sig in the part

@mike-spa Since you have worked before on issues that turned out to be related to deleting C major key signatures at the beginning of a score, could you please check if this PR looks good to you?

[mike-spa]

@cbjeukendrup as far as I can see, this looks perfectly coherent with all the other similar fixes I've done (removing a keySig from the score just because it happens to be C is a very wrong idea IMO, so I'm very glad to see it gone).

[RomanPudashkin]

@cbjeukendrup please pay attention to the failed tests

[cbjeukendrup]

@RomanPudashkin Done!