Instead of using a list of left/right-hand states and an overrideable global
callback, use a map of action and key-list pairs. This makes the plugin much
more portable, does not require any hardware-specific knowledge within the
plugin, and does not require us to treat the hands separately.
This in turn, results in a friendlier user interface, at the cost of limiting
the maximum length of a combination to five keys. A small price to pay.
Signed-off-by: Gergely Nagy <algernon@keyboard.io>