Elements not scaling with staff size

1) load score from https://musescore.org/en/node/284440#comment-897560

Result: many of the markings on the small staves remain full size. This is primarily because these types do not override mag() to account for staff magnification. While in theory we could override Element::mag() to do this, it would actually be a bit of a hack as different element types would need to do this in different ways. So, I'm going in and doing the overrides. There are also a few places we aren't applying staff magnification where we would need to, mostly to style settings like line thicknesses.

This came up in #284440: Positions above/below staff not scaling with staff size but I'm separating out the size from the position issues, as I plan to submit a PR for this issue but might want to defer to someone else for the other, as it could be more "architectural" in nature.