updates midi functionality (#331)

updates midi functionality (#331)

* 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

* splits up process_action to allow independent processing of actions

* moves midi stuff to quantum.c

* adds additional scales for midi