* lily/break-align-interface.cc (self_align_callback): new

* lily/break-align-interface.cc (self_align_callback): new
interface, new function. Look at complete alignment. This handles
tunable break alignments in case break-align-symbols are missing.

* lily/bar-number-engraver.cc (acknowledge_break_alignment): new function

* lily/mark-engraver.cc (acknowledge_break_alignment): new function.

* python/convertrules.py (conv): mark/bar number alignment rule.

* lily/break-align-interface.cc (calc_positioning_done): also
store offset from last visible break-alignment to its
neighbor. This makes alignment on non-visible objects more reliable.