Curvatron - now with Linux support

Finally made Linux support officially available. Some of you might have noticed it was already possible to download the Linux version, but it wouldn't run through Steam, and even if you ran it manually, it had some major problems. It now runs as intended, but some people will still run into some issues, unfortunately. The main game features (all the pre-included levels and game modes) should work fine for everyone, but stuff relating to the Steam platform (community levels, Workshop upload, achievements) may fail. Hopefully this will let more people enjoy our game. Have fun ːpointː