* lily/note-collision.cc (check_meshing_chords): if merging heads,

* lily/note-collision.cc (check_meshing_chords): if merging heads,
then kill the dots too.

* lily/system.cc (output_lines): only junk only-prebreak-interface grobs.

* lily/system.cc (output_lines): don't suicide Spacing_items.

* lily/dot-column.cc (dot_config_badness): new function:
select the best scoring dot configuration: dots should go close to
the note heads, but be shifted up or down according to conventions.
(print_dot_configuration): idem.
(shift_one): idem
(remove_collision): idem.