Issue 5111: Implement spacing-pair for MeasureCounter

Since MeasureCounter grobs are bounded by columns, sophisticated
horizontal positioning is available relative to "prefatory
materials" (such as barlines, clefs, and key signatures) at the
beginning and end of measures.  This allows numbers to be
placed above full-measure rests which have been similarly adjusted
with 'spacing-pair.

The previous default is established in the grob description: count
objects are centered between BreakAlignment grobs.

Paper_column::break_align_width is made available to Scheme as
ly:paper-column:break-align-width.

A regression test is provided.