(acknowledge_system_start_delimiter): use
add_offset_callback(). This fixes translation of nested
staffgroups.
* lily/grob-closure.cc (add_offset_callback): only encaps in
simple_closure if it's a procedure.
* lily/span-bar.cc (calc_glyph_name): read glyph-name, not glyph
from bar-line.
* ly/engraver-init.ly (RemoveEmptyRhythmicStaffContext): add
remove-empty where pertinent.