Fix #1192.

Fix #1192.

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

* scm/define-context-properties.scm (all-user-translation-properties):

  remove verticallySpacedContexts definition

* scm/define-grob-interfaces.scm:

  remove vertically-spaceable-interface

* scm/define-grob-properties.scm (all-internal-grob-properties):

  remove 'spaceable-staves definition

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

  remove vertically-spaceable-interface from VerticalAxisGroup