⌂ Display bracket with only one staff in a system

If there is only one staff in one of the staff types ChoirStaff or StaffGroup, by default the bracket and the starting bar line will not be displayed. This can be changed by overriding collapse-height to set its value to be less than the number of staff lines in the staff.

Note that in contexts such as PianoStaff and GrandStaff where the systems begin with a brace instead of a bracket, another property has to be set, as shown on the second system in the example.