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