Refactored the code a little, so all callbacks now receive a `user_data`
pointer, which can be anything. As an example, the key pairs from
`ACTION_TAP_DANCE_DOUBLE` now use this, and custom, built-in functions.
This makes it easier to extend the tap dance functionality, and also
simplifies the code a little.
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>