Changes to consider when updating the file format

We don't get the chance to make breaking changes to the file format that often, so when we do get such a chance, we should fully utilise it. Therefore I thought it would be good to build a list together of things we would like to consider changing.

• Rename the instrumentId property to musicXmlId, which is far more accurate (in the code it has already been renamed)
• Maybe things necessary for "not-yet-fully-created" parts, see #15546 (comment)
• Maybe: write the main mscx file under a fixed filename inside the mscz. Would probably have prevented the two "peculiar cases" in #15739. Disadvantage: possibly less convenient for uncompressed folders.
• pedalLineStyle typo: #12133
• A similar list on MuseScore.org: https://musescore.org/en/node/296654
• https://github.com/musescore/MuseScore/pull/16369#issuecomment-1475124828
• ...