* lily/separating-line-group-engraver.cc (acknowledge_grob):

* lily/separating-line-group-engraver.cc (acknowledge_grob):
set/unset breakableSeparationItem

* lily/separation-item.cc (relative_width): add function.

* scm/define-grob-interfaces.scm (only-prebreak-interface): add
only-prebreak-interface

* lily/system.cc (output_lines): only junk only-prebreak-interface grobs.

* lily/multi-measure-rest-engraver.cc (stop_translation_timestep):
read breakableSeparationItem

* input/regression/multi-measure-rest-multi-staff-center.ly: new file.

* lily/system.cc (output_lines): don't suicide Spacing_items.