Music themes

First, we want to play background music depending on the state we are currently in. We have prepared two themes: One for the menu, and one for the game itself. We'll define a corresponding enum:

namespace Music
{
    enum ID
    {
        MenuTheme,
        MissionTheme,
    };
}

We'll then create a class that has an interface dedicated to music playing:

class MusicPlayer : private sf::NonCopyable
{
    public:
                            MusicPlayer();

        void                play(Music::ID theme);
        void                stop();

        void                setPaused(bool paused);
        void                setVolume(float volume);

    private:
        sf::Music                           mMusic;
        std::map<Music::ID, std::string>    mFilenames;
        float                               mVolume;
};

The method names should be self-explanatory. We have a single sf::Music instance that represents the currently-played music. The mFilenames variable maps music...