[MU3] Fix #314935 - Adding instruments on top of others whose parts have be…

[njvdberg]

Resolves: https://musescore.org/en/node/314935

When adding/removing or changing the order of staves, for all Excerpts the track mapping between Excerpt and masterScore must be updated but the original implementation had some flaws.

• 1 The rebuild of the mapping was incomplete
• 2 The rebuild of the mapping was in MuseScore::editInstrList() which also adds/removes staves. However this adding/removing was done via the "undo" mechanism, the rebuild if the mapping was't.

PR #7029 moved the rebuild of the mapping inside the "undo" class SortStaves() which used when the order of the staves was changed.

This PR moves the rebuild of the mapping into a nwe "undo" class MapExcerptTracks() and inside MuseScore::editInstrList() a new instance of the class is created and added to the undo stack. As a result, the mapping will always adapted to the new staff order.

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