* scm/define-context-properties.scm

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

* scm/define-grob-interfaces.scm (vertically-spaceable-interface):
add interface

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

* scm/define-grobs.scm (all-grob-descriptions): set
vertically-spaceable-interface for VerticalAxisGroup and
RemoveEmptyVerticalGroup

* lily/system.cc (get_line): determine staff_refpoints_
using spaceable-staves.

* lily/score-engraver.cc (acknowledge_grob): ack
vertically-spaceable-interface and verticallySpacedContexts to
set spaceable-staves.

* input/regression/new-slur.ly: add slur under 16th subbeam case.

* lily/slur-scoring.cc (fill): init has_same_beam_ before use.