added GameSystem entity

MediaBrowser.Controller/Entities/GameSystem.cs

@@ -0,0 +1,23 @@
+using System;
+
+namespace MediaBrowser.Controller.Entities
+{
+    /// <summary>
+    /// Class GameSystem
+    /// </summary>
+    public class GameSystem : Folder
+    {
+        /// <summary>
+        /// Return the id that should be used to key display prefs for this item.
+        /// Default is based on the type for everything except actual generic folders.
+        /// </summary>
+        /// <value>The display prefs id.</value>
+        public override Guid DisplayPreferencesId
+        {
+            get
+            {
+                return Id;
+            }
+        }
+    }
+}