Add vertical separation to audio bus

[passivestar]

Makes spacing more consistent with the rest of the editor. "6" almost touching the top line was especially noticeable

Before	After

[Mickeon]

Personal peeve, although the separation is good, I think it's a bit too much. Although it really puts to question the existing design, the effects list has even less breathing room in this PR.

[passivestar]

Personal peeve, although the separation is good, I think it's a bit too much. Although it really puts to question the existing design, the effects list has even less breathing room in this PR

It's around the same size as other gaps:

I think if the effects list is a concern the volume meter should be shortened, not the gaps. Inconsistent gaps look scuffed

[akien-mga]

Thanks!