Fix #309419: instrument change data not properly saved in linked staves

[elerouxx]

Resolves: https://musescore.org/en/node/309419

Problem: Editing a Staff instrument on linked staves, after an instrument change, only changed the instrument for the first InstrumentChange element (the first staff). The changes apparently worked, but they were lost after saving and reloading the file (because the second staff's instrument change was written with the stock unchanged instrument).

Just added a loop in Edit Staff so it now finds all Instrument Changes on the part and its staves and sets all of them to the new instrument data.

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