#4938 Refactor handling of supported MIDI control changes

Add Audio_item and Midi_item subclasses for (MIDI control, value) changes


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.


Rename Midi_control_function_performer to Midi_control_change_performer

[ht, 2016-07-20 19:24:28]

This patch set separates the code refactoring originally done for Issue 4745
from the enhancements implemented as part of that issue. The patch does not
change any existing functionality (nor introduce any new features), its purpose
is to only improve the maintainability of the current implementation by
simplifying the interfaces, removing duplication of code, and cleaning up some
terminology.