Added logging of SDL system versions

Added logging of SDL system (compile & runtime) version information.
    src/core/MainManager.cpp
    src/core/MainManager.h
Added basic music loading and playback

Added basic music loading and playback
    src/audio/AudioPlayback.cpp (+)
    src/audio/AudioPlayback.h   (+)
    src/audio/MusicPlayer.cpp   (+)
    src/audio/MusicPlayer.h     (+)
    src/audio/MusicTrack.cpp    (+)
    src/audio/MusicTrack.h      (+)
    src/ui/SDL_mixer.h -> src/audio/SDL_mixer.h
    src/core/MainManager.cpp
    src/io/ResourceManager.cpp
    src/io/ResourceManager.h
    src/util/Asset.cpp
    src/util/Asset.h
    src/util/TestAsset.h