Move to SDL_mixer for Windows builds?

[Hypexion]

Currently, Windows builds of DROD uses FMOD as a sound library. This has a few potential issues:

• FMOD is a proprietary library. Indeed, it's the only library DROD uses that isn't open source
• The current version of FMOD used by DROD is very outdated, so much so that it isn't available directly via FMOD
• Having to maintain two versions of the sound system for different platforms is annoying

In terms of building switching to SDL_mixer is (hopeful just) a case of setting a compiler flag. In terms of distribution, it requires the SDL_mixer dll to be inlcuded in installers and store packages.