* input/test/piano-staff-distance.ly: new file.

* input/test/piano-staff-distance.ly: new file.

* lily/translator-group.cc (recurse_down_translators): use
Direction for bottom-up/top-down.

* lily/include/translator.hh (class Translator): add
process_music() and do_announces(). This obviates
recurse_down_{engravers,performers}.

* ly/declarations-init.ly (melismaEnd): use ManualMelismaEvent for
\melisma and \melismaEnd

* lily/melisma-engraver.cc (process_music): change to
Melisma_translator, unify with performer. Accept
ManualMelismaEvent.

* ly/engraver-init.ly: add Font_size_engraver.