Add a register/unregister_code16 pair of functions

Add a register/unregister_code16 pair of functions

These functions register not only the 8bit keycode, but the modifiers
too. It doesn't handle the full range of the upper 8bits, just the mods,
but that's a good start.

Changed the tap-dance pair functions to use these, so one can do:

  `ACTION_TAP_DANCE_DOUBLE (KC_COLN, KC_SCLN)`

...and that will do the right thing.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>