Rename ThreeState -> TernaryState and add basic tests

osu.Game/Graphics/UserInterface/TernaryState.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 TernaryState
+    {
+        /// <summary>
+        /// The current state is false.
+        /// </summary>
+        False,
+
+        /// <summary>
+        /// The current state is a combination of <see cref="False"/> and <see cref="True"/>.
+        /// The state becomes <see cref="True"/> if the <see cref="ThreeStateMenuItem"/> is pressed.
+        /// </summary>
+        Indeterminate,
+
+        /// <summary>
+        /// The current state is true.
+        /// </summary>
+        True
+    }
+}