Support controller touchpads

SDL has an API for controller touchpads: https://github.com/libsdl-org/SDL/blob/main/include/SDL_events.h#L430

These are mostly present on PS4 and PS5 controllers, but notably Steam Deck will have two of them.

I don't think OpenMW itself should allow doing anything with touchpads, but exposing them to the Lua API will be very useful.