* ly/engraver-init.ly (AncientRemoveEmptyStaffContext): replace
System_start_delimiter_engraver by Nested_system_start_delimiter_engraver.
* lily/nested-system-start-delimiter-engraver.cc (struct
Bracket_nesting_node): new class.
(struct Bracket_nesting_group): new class
(struct Bracket_nesting_staff): new class.
(process_music): create hierarchy of grobs. This allows separate
tuning of different SSDs.
* scm/define-grobs.scm (all-grob-descriptions): new Grob SystemStartSquare
(all-grob-descriptions): remove old NestedSystemStartDelimiter.