Access wlr_seat [input]_state structs

[acrisci]

https://github.com/swaywm/wlroots/blob/d3670dfeff43cbde98fae47fdb60ef223ce8c220/include/wlr/types/wlr_seat.h#L127

Access to these will be needed for some input related things. For instance, client cursors needs to know what the surface with pointer focus is so it knows whether the set cursor request is valid.

[Timidger]

Ok. For the seat and surface we should return SeatHandle and SurfaceHandles.

Also one thing that has a struct but isn't actually wrapped is wlr_seat_client since I wasn't actually sure how it related to the seat. Seems there's documentation there though #32 should be finished soon (though this is a separate issue, just wanted to bring it up)