Adjust volume on a per-highlight basis

[jupjohn]

Individuals may want to make certain highlights more of a priority than others while keeping the same notification sound. Instead of having to tweak the sound for each highlight at the file level (or find the default c2 sound and modify it), it would be a great improvement to have a volume modifier on the highlight entry itself.

I'm aware the highlight panel is already crowded - I'll try to think of a way to incorporate it or shuffle things around.
This isn't a duplicate of #1436

[8thony]

This was moved into an issue #7380.