Integrate video detail and audio settings into preferences

[Quintus]

This code in preferences.cpp:

    // FIXME: Merge these settings with the other ordinary settings
    // in a cPreferences instance! The following lines set global
    // variables, which from the outside is totally unexpected.
    // This must be done in main.cpp instead, where the preferences
    // are loaded!
    pVideo->m_geometry_quality = loader.Get_Video_Geometry_Detail();
    pVideo->m_texture_quality  = loader.Get_Video_Texture_Detail();
    pAudio->m_music_volume     = loader.Get_Audio_Music_Volume();
    pAudio->m_sound_volume     = loader.Get_Audio_Sound_Volume();

sets global variables where it is totally unexpected from the outside. The 4 relevant settings must be included into cPreferences and the corresponding globals must be set in the calling context in main.cpp on game startup.