`SeatHandler::new_seat` not called in `SeatState::new`

It seems like SeatHandler::new_seat is only called when new globals are added at runtime, and not when SeatState::new is called :/

This doesn't seem to be a bug, but just being due to `Dispatch` not having an `on_bind` method or similar.

For now the workaround seems to be to apply `new_seat` manually.