Better structure for song sorter and bugfix for "DateAdded" sorting

VocaluxeLib/Enums.cs

@@ -189,7 +189,6 @@ namespace VocaluxeLib
     {
         // ReSharper disable InconsistentNaming
         TR_CONFIG_NONE,
-        //TR_CONFIG_RANDOM,
         TR_CONFIG_FOLDER,
         TR_CONFIG_ARTIST,
         TR_CONFIG_ARTIST_LETTER,

VocaluxeLib/IBasic.cs

@@ -279,7 +279,7 @@ namespace VocaluxeLib
     public interface IDataBase
     {
         bool GetCover(string fileName, ref CTexture texture, int coverSize);
-        bool GetDataBaseSongInfos(string artist, string title, out int numPlayed, out string dateAdded, out int highscoreID);
+        bool GetDataBaseSongInfos(string artist, string title, out int numPlayed, out DateTime dateAdded, out int highscoreID);
     }
 
     public interface IControllers

VocaluxeLib/Songs/CSong.cs

@@ -172,7 +172,7 @@ namespace VocaluxeLib.Songs
         public readonly List<string> Languages = new List<string>();
 
         public int DataBaseSongID = -1;
-        public string DateAdded = "";
+        public DateTime DateAdded = DateTime.Today;
         public int NumPlayed;
         public int NumPlayedSession;