Add some sound and give keyboard more control

When I tried hosting a game on Parsec, I wasn't able to access the overlay with the mouse because I had disabled the mouse cursor, so now it reappears when you move the mouse.

It was too easy for any player to pause or abort a game, or even exit the program entirely, so now the ESC key on the keyboard is the only way to do any of that.

It felt a bit barren with no sound, but at least now there's a bit of sound when a player drops mass.

I also added Linux and OS X builds, but I don't even know if they work yet.