Make skip button work.

osu.Game/Screens/Play/KeyCounterMouse.cs

@@ -0,0 +1,33 @@
+//Copyright (c) 2007-2016 ppy Pty Ltd <contact@ppy.sh>.
+//Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using osu.Framework.Graphics;
+using osu.Framework.Input;
+using OpenTK;
+using OpenTK.Input;
+
+namespace osu.Game.Screens.Play
+{
+    public class KeyCounterMouse : KeyCounter
+    {
+        public MouseButton Button { get; }
+        public KeyCounterMouse(string name, MouseButton button) : base(name)
+        {
+            Button = button;
+        }
+
+        public override bool Contains(Vector2 screenSpacePos) => true;
+
+        protected override bool OnMouseDown(InputState state, MouseDownEventArgs args)
+        {
+            if (args.Button == this.Button) IsLit = true;
+            return base.OnMouseDown(state, args);
+        }
+
+        protected override bool OnMouseUp(InputState state, MouseUpEventArgs args)
+        {
+            if (args.Button == this.Button) IsLit = false;
+            return base.OnMouseUp(state, args);
+        }
+    }
+}