* 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
* merging?
If one wants to temporarily disable the key cache (for example because
it interferes with a macro), `disable_action_cache` must be set to
`true`. `process_action_nocache` is a simple wrapper doing just that for
a single call.
