System separator positioning depends on correct calculation of the
paper-system property 'staff-refpoint-extent, but this is incorrect for
systems which include Staff-like contexts since the
Vertically_spaced_contexts_engraver fails to acknowledge axis-groups whose
origin context is aliased to Staff.
Though it's simple to enhance the axis-group acknowledger to detect aliased
contexts, a better solution is to remove the Vertically_spaced_context_engraver:
spaceability is now determined by 'staff-affinity, so the engraver's more
limited functionality can be folded into System::get_paper_system ().
* lily/system.cc (get_paper_system):
calculate 'staff-refpoint-extent via VerticalAlignment and
Page_layout_problem::is_spaceable ()
remove 'spaceable-staves
* lily/vertically-spaced-context-engraver.cc:
delete
* ly/engraver-init.ly:
remove Vertically_spaced_context_engraver and verticallySpacedContexts