add _keyList property to masterscore

Your idea

Key signature is (in most cases) global property of system. Local key signature for single staff is exception of this general rule.

So it make sense to have master score _keyList property, where global key signatures are stored.

Similar, like we have _timemap and _tempomap.

It would solve some problems, namely:

• if first staff has local key signature in place, where global key signature is in score, newly added part copies this local key signature, instead of correct global key signature

https://github.com/musescore/MuseScore/assets/1646034/217c1af6-930a-4189-be32-482576edb149

• if all instruments are removed in Instruments tab, key list for score is lost - newly added instrument has no key signatures (but has time signatures)

https://github.com/musescore/MuseScore/assets/1646034/857d4091-2305-4866-8fae-fdad6e3fe2b7

Problem to be solved

Properly define, what "global key signature is", how it is added to score (standard key signature for all staves), how is removed, ...

Rules should be consistent with time signatures (masterscore._timemap).

Prior art

No response

Additional context

Once this FR would be aplied, #18949 wouldn't be relevant anymore and may be reverted.
Some context - #18906.

Start by reading the existing masterscore._timemap and _tempomap behavior, then review the related issues #18949 and #18906. The issue considers how a master-score key list should define, add, and remove global key signatures; done would mean those rules and the affected part-creation and instrument-removal cases are consistently addressed.