CodaMark and friends not included in skyline

[Werner LEMBERG]

I noticed that the following grobs are not part of the skyline.
Actually, I would expect that they are part of it...

  BarNumber
  CodaMark
  MetronomeMark
  RehearsalMark
  SectionLabel

Interestingly, `SegnoMark` and `JumpScript` are both part of the
skyline.  The same is true for `CenteredBarNumber`.

What's the logic behind it?  IMHO, it should be made consistent.  And
it should be documented, too.


    Werner