fix #308726: Avoid rest offsets with slash notation

[tobik]

Resolves: https://musescore.org/en/node/308726

Do not offset rests if slash notation is active anywhere in the chord

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[MarcSabatella]

See https://musescore.org/en/node/318800 - I think this probably is what broke rests within the rhythmic slash notation. If I'm following this correctly, the original problem was that rests not in slash notation were being offset unnecessarily. Now, unfortunately, the rests that are in slash are not being offset either, and they definitely need to be (and were previously).

Probably the code added here needs to be skipped if the current rest is itself in slash notation. Well, at least if in voice 3 or 4, which are the ones that need to be offset.