* python/midi.c (midi_parse_track): robustness: don't read past

* python/midi.c (midi_parse_track): robustness: don't read past
end of string.

* ly/performer-init.ly: add Control_track_performer, move
Tempo_performer and Time_signature_performer to Score.

* lily/score-performer.cc (acknowledge_audio_elements): override
from base class: add to audio-columns

* lily/control-track-performer.cc (add_text): new file: generate
the control track.

* lily/performance.cc: move output_header_track to Control_track_performer()

* lily/midi-walker.cc (Midi_walker): get channel in constructor.

* lily/include/midi-item.hh (class Midi_channel_item): insert
class into hierarchy, for items that can have a channel setting. Dehungarify.

* lily/include/performer.hh (class Performer): remove
play_element();  move functionality into announce/acknowledge.

* lily/audio-staff.cc (output): remove channel_ from Midi_track.