Issue 4938 (2/3) Refactor handling of MIDI control changes

Issue 4938 (2/3) Refactor handling of MIDI control changes

Handle the MIDI control value initialization from context properties
(Staff_performer::new_audio_staff), control value changes
(Midi_control_function_performer::announce_function_value_change), and
value conversion for output
(Midi_control_function_value_change::to_string) in the new
Midi_control_change_announcer class.

All MIDI control changes are now encoded using
{Audio,Midi}_control_change items.  This change makes the old
{Audio,Midi}_control_function_value_change classes obsolete.