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/Graphics.cpp

@@ -3280,6 +3280,7 @@ void Graphics::reloadresources()
 		screenbuffer->LoadIcon();
 	}
 
+	music.destroy();
 	music.init();
 }