Fix #12530 Apply !showbacktied behavior per-system rather than per-measure (restore MU3 behavior)

[asattely]

Resolves: https://github.com/musescore/MuseScore/issues/12530

See discussion in the linked issue for details.

There is a lot of discussion about what should and should not be shown if this option is turned on or off. Ultimately, there will be some kind of style setting that controls this, but for now we will use MU3's defaults.

[vpereverzev]

@Tantacrul quite a risky one, bumping to post-release, since it's a P3 anyway

[RomanPudashkin]

any new tests?