Separate ThreeStates into its own file

osu.Game/Graphics/UserInterface/ThreeStateMenuItem.cs

@@ -76,23 +76,4 @@ namespace osu.Game.Graphics.UserInterface
             }
         }
     }
-
-    public enum ThreeStates
-    {
-        /// <summary>
-        /// The current state is disabled.
-        /// </summary>
-        Disabled,
-
-        /// <summary>
-        /// The current state is a combination of <see cref="Disabled"/> and <see cref="Enabled"/>.
-        /// The state becomes <see cref="Enabled"/> if the <see cref="ThreeStateMenuItem"/> is pressed.
-        /// </summary>
-        Indeterminate,
-
-        /// <summary>
-        /// The current state is enabled.
-        /// </summary>
-        Enabled
-    }
 }

osu.Game/Graphics/UserInterface/ThreeStates.cs

@@ -0,0 +1,27 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+namespace osu.Game.Graphics.UserInterface
+{
+    /// <summary>
+    /// An on/off state with an extra indeterminate state.
+    /// </summary>
+    public enum ThreeStates
+    {
+        /// <summary>
+        /// The current state is disabled.
+        /// </summary>
+        Disabled,
+
+        /// <summary>
+        /// The current state is a combination of <see cref="Disabled"/> and <see cref="Enabled"/>.
+        /// The state becomes <see cref="Enabled"/> if the <see cref="ThreeStateMenuItem"/> is pressed.
+        /// </summary>
+        Indeterminate,
+
+        /// <summary>
+        /// The current state is enabled.
+        /// </summary>
+        Enabled
+    }
+}