* scm/define-grobs.scm (all-grob-descriptions): new grob

* scm/define-grobs.scm (all-grob-descriptions): new grob
NestedSystemStartDelimiter.

* input/regression/system-start-heavy-bar.ly: new file.

* lily/nested-system-start-delimiter-engraver.cc: new file.

* lily/nested-system-start-delimiter.cc: new file.

* input/regression/system-start-nesting.ly: new file.

* scm/define-grobs.scm (all-grob-descriptions): use X side
positioning for positioning braces for nested contexts.

* lily/system-start-delimiter.cc (print): don't divide extent by
staff_space.
(print): use style iso. glyph.
(staff_bracket): translate glyph inside routine already.