Move the mute logic to musicclass.

This moves the last of the SDL_mixer calls to Music.cpp.

desktop_version/src/Music.h

@@ -64,6 +64,7 @@ public:
     void resumeef(void);
 
     bool halted(void);
+    void updatemutestate(void);
 
     std::vector<SoundTrack> soundTracks;
     std::vector<MusicTrack> musicTracks;