Updating default shortcuts should work without shortcuts needing to be reset

[avvvvve]

Issue

If a user has previously customized at least one shortcut, a shortcuts.xml file is created in /Users/username/Library/Application Support/MuseScore/MuseScore4/ (or .../MuseScore4Development in a dev build).

It seems that sometimes (or all the time?) if we then want to edit the new default shortcut for an action, the presence of the shortcuts.xml file prevents all shortcuts from being updated. So if a user wants the new shortcut, they have to either wipe all their custom shortcuts (either by deleting the .xml file or selecting all the shortcuts in the Preferences table and resetting them) or finding the individual action(s) in the preferences list to reset, which can be difficult.

Ideally in this scenario, only shortcuts that a user has already customized would be left alone, and we could overwrite any shortcuts in the shortcuts.xml file that match the previous version's default. Or we could "tag" user customized shortcuts in the .xml file some other way so we know not to overwrite them.

Additional context from @cbjeukendrup here:

I realised this is a bit of a special case of the shortcuts issue, because here we're adding more shortcuts to an action that already had shortcuts previously. So new shortcuts for new actions (or actions that didn't previously have shortcuts) work fine, while new shortcuts for actions that already had shortcuts previously don't work without resetting the shortcuts for those actions.

[bkunda]

If I understand this correctly, this task is required to ensure new shortcut assignments (like this one) work correctly when users update to the new app version.