Smooth scroll events via touchpad

This enables handling of `GDK_SCROLL_SMOOTH` events.
When enabled via "smooth-scroll" config option will generate events of type `GIRARA_EVENT_SCROLL_BIDIRECTIONAL` with motion data in `girara_argument_t` as `double[2]`.
This needs to be added after GIRARA_EVENT_OTHER. Yes, it would make more sense here. But it changes the value of GIRARA_EVENT_OTHER and thus the ABI.