Add alternative for random beatmap selection

"Never repeat" will not repeat until all songs have been seen by repeatedly pressing F2/Random button

osu.Game/Configuration/SelectionRandomType.cs

@@ -0,0 +1,15 @@
+// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System.ComponentModel;
+
+namespace osu.Game.Configuration
+{
+   public enum SelectionRandomType
+    {
+        [Description("Random")]
+        Random,
+        [Description("Never repeat")]
+        RandomPermutation
+    }
+}