Fix #915.
* lily/bar-line.cc:
non_empty_barline () moved from lily/note-spacing.cc
* lily/multi-measure-rest.cc (bar_width):
read 'spacing-pair to determine spacing to left and right columns instead
of fixed spacing to BreakAlignment for both directions
* lily/note-spacing.cc (get_spacing):
non_empty_barline -> Bar_line::non_empty_barline
* lily/paper-column.cc (break_align_width):
add alignment symbol arg and use it to find correct grob
* scm/define-grob-properties.scm (all-user-grob-properties):
doc 'spacing-pair
* scm/define-grobs.scm (all-grob-descriptions):
add 'spacing-pair defaults for MultiMeasureRest and PercentRepeat