* Documentation/topdocs/NEWS.tely (Top): add entry for percent

* Documentation/topdocs/NEWS.tely (Top): add entry for percent
counters.

* Documentation/user/basic-notation.itely (Measure repeats): add
links to Counter grobs.

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

* lily/percent-repeat-engraver.cc (class Percent_repeat_engraver):
also create counter grobs.

* lily/separation-item.cc (extremal_break_aligned_grob): don't
return grob without space-alist.

* input/proportional.ly: tune staff-padding.

* input/regression/repeat-percent-count.ly: new file.