Keep a staff alive with multiple layers (issue 308910043 by address@hidden)

[mark . opus11]

Reviewers: ,

Description:
Keep a staff alive with multiple layers

This allows the `VerticalAxisGroup.remove-layer' property to
accept a list of values. The layer will stay alive with any
other member of the Keep_alive_together_engrave group with a
remove-layer value in that list.

The principal reason for this patch was to allow the use of
MarkLine contexts in a Frenched score, where the context should
stay alive with any single staff in a StaffGroup. This
implementation should also allow additional flexibility with ossia
and divisi staves.

Please review this at https://codereview.appspot.com/308910043/

Affected files (+208, -4 lines):
  A Documentation/snippets/new/using-marklines-in-a-frenched-score.ly
  A input/regression/remove-layer-list.ly
  M lily/keep-alive-together-engraver.cc
  M scm/define-grob-properties.scm