Implement shuffle (not surfaced)

RedBookPlayer.GUI/ViewModels/PlayerViewModel.cs

@@ -470,6 +470,11 @@ namespace RedBookPlayer.GUI.ViewModels
         /// </summary>
         public void ExecutePreviousTrack() => _player?.PreviousTrack();
 
+        /// <summary>
+        /// Shuffle the current track list
+        /// </summary>
+        public void ExecuteShuffle() => _player?.ShuffleTracks();
+
         /// <summary>
         /// Move to the next index
         /// </summary>