adds a sequencer to the music mode (#330)

* implements leader key for planck experimental

* allows override of leader timeout

* adds ability to use the leader key in seq

* fixes leader keycode

* adds chording prototype

* fixes keycode detection

* moves music mode to quantum.c

* disables chording by default

* adds music sequencer functionality

* implements audio/music functions in quantum.c

* Merge branch 'master' into process-record

quantum/audio/audio.h

@@ -29,6 +29,7 @@ bool is_audio_on(void);
 void audio_toggle(void);
 void audio_on(void);
 void audio_off(void);
+void audio_on_callback(void);
 
 // Vibrato rate functions