Keep a staff alive with multiple layers

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.