Sound and Input cleanup

This is just some house-keeping. Essentially a followup of https://github.com/OpenRA/OpenRA/pull/5235 and https://github.com/OpenRA/OpenRA/pull/5248. The organisation in this regard has been historically weird. I believe this arranges it better without making too many changes to the overall structure. Creating a new OpenRA.Platform.dll as proposed in https://github.com/OpenRA/OpenRA/issues/5274 is a lot more trouble than it is worth. I tried it at first, but there are still a lot of ties back and forth. We also don't support an SDL2/OpenTK alternative.