Introduce back the concept of Rulesets.

osu.Game/GameModes/Play/Ruleset.cs

@@ -0,0 +1,38 @@
+//Copyright (c) 2007-2016 ppy Pty Ltd <contact@ppy.sh>.
+//Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using osu.Game.Beatmaps.Objects;
+using osu.Game.GameModes.Play.Catch;
+using osu.Game.GameModes.Play.Mania;
+using osu.Game.GameModes.Play.Osu;
+using osu.Game.GameModes.Play.Taiko;
+
+namespace osu.Game.GameModes.Play
+{
+    public abstract class Ruleset
+    {
+        public abstract ScoreOverlay CreateScoreOverlay();
+
+        public abstract HitRenderer CreateHitRendererWith(List<HitObject> objects);
+
+        public static Ruleset GetRuleset(PlayMode mode)
+        {
+            switch (mode)
+            {
+                default:
+                    return new OsuRuleset();
+                case PlayMode.Catch:
+                    return new CatchRuleset();
+                case PlayMode.Mania:
+                    return new ManiaRuleset();
+                case PlayMode.Taiko:
+                    return new TaikoRuleset();
+            }
+        }
+    }
+}