* python/convertrules.py (conv): warning on \tempo{}

* python/convertrules.py (conv): warning on \tempo{}

* ly/performer-init.ly: set tempoWholesPerMinute.

* ly/midi-init.ly: remove \midi

* lily/tempo-performer.cc: look at tempoWholesPerMinute to set
MIDI tempo.

* lily/metronome-engraver.cc (process_music): use tempoUnitCount
tempoUnitDuration for determining what to print.

* lily/lyric-extender.cc: typo.

* lily/parser.yy (output_def_body): disallow \tempo in \midi{}

* lily/duration-scheme.cc (LY_DEFINE): ly:duration-length: new function.

* THANKS: update sponsors.

* ly/english.ly: quarter tone naming (thanks, Trevor Baca)