Audio mode for each playlist entry (#909)

- Playlist entries get a second select slide for the audio mode (normal, instrumental, vocals, karaoke)
- The slide is only shown if the song has an instrumental file, otherwise normal is used
- The audio mode is saved in the playlist file and used for each round
- Sing, score and highscore screens use the audio mode of the current round
- Song options select the audio mode by mode instead of position
- ScreenSong theme version 15

Co-authored-by: Florian Ostertag <flokuep@users.noreply.github.com>

VocaluxeLib/XmlStructs.cs

@@ -26,5 +26,7 @@ namespace VocaluxeLib
         public string Artist, Title;
         [DefaultValue(EGameMode.TR_GAMEMODE_NORMAL)]
         public EGameMode GameMode;
+        [DefaultValue(EAudioMode.TR_AUDIOMODE_NORMAL)]
+        public EAudioMode AudioMode;
     }
 }