Defining the staves to be included in barlines

[notator]

This could be done using system staff indices, in the way bracket heights are defined in #277.
Proposal: Define the following two new elements:

• <barline>:
  • Attributes: top-staff and bottom-staff. These are the top and bottom system staff indices of the staves included in the barline. Barline types are defined in <measure-global>.
  • Content: none
  • Parent element <barlines>
• <barlines>:
  • Attributes: none
  • Content: one or more <barline> elements
  • Parent element: <system-layout>. <barlines> should be defined after the -layout elements in <system-layout>, so that the staves can be indexed properly.

[clnoel]

This could also be done inside our current system-group-staff layout hierarchy by adding the following attribute to <group-layout>:

barline: enum staff, group, mensurstrich

• staff (default): put the barlines only on the staves themselves.
• group: put the barline from the top of the group to the bottom of the group, including the spaces in between
• mensurstrich: put the barline only between staves in the group.

This works because there is a group-layout symbol option of none, which has no visible left-side bracket or brace, but which can still group staves, in the case where we want the barlines to do something that the left-side grouping symbols are not.