Fix part rename not persisting for potential excerpts

[manolo]

Resolves: #31656

When renaming a part in the Parts dialog, the new name wasn't persisted to the .mscz file unless the part had been opened as a tab first.

Root cause: "Potential" excerpts shown in the Parts dialog are not in masterScore()->excerpts() until explicitly created. Since mscsaver only saves excerpts in the master's list, renaming a potential excerpt had no effect.

Solution: ChangeExcerptTitle now checks if the excerpt is in the master's list before renaming. If not, it calls initAndAddExcerpt() to add it, ensuring it gets saved.

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[cbjeukendrup]

This is not ideal, because initialising an excerpt is an expensive operation, and it is wasteful to populate and store the entire excerptScore while we only really need to store the excerpt's name.

Instead, the solution to this and some other issues, should be to move the concept of "potential excerpts" to the engraving/dom level, instead of having it at the MasterNotation/PartListModel level.
Potential excerpts should just be Excerpt but with no Score yet. Such excerpts should be created for each instrument when the master score is initially created, and when new instruments are added.
They should be stored in the MSCZ file, even if no properties have been modified compared to the defaults. However, the file format for potential excerpts might need to be slightly different because of the absence of a Score. It should only store a name and a list of Parts.

This is a somewhat big refactor, but I think it's worth it. It would also result in the ability to delete potential excerpts without having them appear back next time you open the parts dialog, and it would make it easier to fix the problems with the reset functionality.

Let us know if you're happy to look into this!