This is an inelegant hack for #2522 but makes things work. Basically we give `action.c` a chance to handle the hold event early so that we can swap the keyboard for later keys. Later, to allow the hold to happen again quickly we nuke the key record so that tapping is reset. I tried to find a cleaner way, honestly.
This adds an action, `ACTION_SWAP_HANDS`, that swaps the the keys on the keyboard across a keymap-defined hemisphere in order to support one-hand typing without requiring a separate one-handed layer. See updated `doc/keymap.md` for more information.
* 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.
