The previous API was inconsistent and different to the rest of libratbag's
API. Replace with one where a macro is a separate struct, can be initialized
and then written to the device with a set_macro() call. The device copies the
contents, the macro is otherwise unmodified and can be re-used by the client
if need be.
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>