Fix collisions between breathing sign and accidentals

This very simple change brings an obvious correctness improvement. However, we might go further and set extra-spacing-height to '(-inf.0 . +inf.0) for BreathingSign. Adjacent columns are allowed to overlap in some cases, like in the regression test spacing-folded-clef3.ly:

This is what was happening in issues #1006 (closed) and #4556 (closed), but the breathing sign was viewed as if it were on the central staff line, leading to collisions. This patch makes the skylines at least accurate. However, I think that from a musical point of view, accidentals should never pass over or under the breathing sign, even when they are sufficiently high or low that no collision arises. In other words, I think that the appearance of the regression test parenthesize.ly is still bad with this patch:

and it should instead look like this, with \override BreathingSign.extra-spacing-height = #'(-inf.0 . +inf.0)

Thoughts? Can anyone confirm this with a reference in engraving literature?

The first commit is an independent cleanup using the facility introduced in commit 0c87b427.