Move musicclass cleanup to separate function musicclass::destroy()

This removes the music cleanup code from musicclass::init(), and
requires that we also call destroy() in Graphics::reloadresources().

This is because we'll need to re-use the musicclass cleanup code
elsewhere, and we don't want to copy-paste the cleanup code. Or at
least, I don't (but I'm not a game dev, game devs copy-paste all the
friggin' time).

desktop_version/src/Music.h

@@ -13,6 +13,7 @@ class musicclass
 public:
 	musicclass();
 	void init();
+	void destroy();
 
 	void play(int t, const double position_sec = 0.0, const int fadein_ms = 3000);
 	void resume(const int fadein_ms = 0);