SeatHandler is not needed

[i509VCB]

SeatHandler's &SeatState is unused and therefore useless.

[Drakulix]

Not really any more the case after #689, no?

[PolyMeilex]

Initially it was done to allow adding SeatState::seats() in the future, and the possibility to and some helpers like, last active seat, or whatever may be useful, So the need for Vec<Seat> in user's code would be mitigated.

My idea was something like this:

seat_state.add_seat(...); // Init new Seat global

seat_state.seats(); // Newly added seat is listed here

seat_state.remove_seat(...) // Disable/Remove Seat Global and remove it from the list

And coincidently Drakulix's changes align with that, as now seat_state is responsible for seat creation.