Add IDeepCloneable interface and update existing CreateCopy methods to use it

osu.Game/Utils/IDeepCloneable.cs

@@ -0,0 +1,16 @@
+// 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.Utils
+{
+    /// <summary>A generic interface for a deeply cloneable type.</summary>
+    /// <typeparam name="T">The type of object to clone.</typeparam>
+    public interface IDeepCloneable<out T> where T : class
+    {
+        /// <summary>
+        /// Creates a new <typeparamref name="T" /> that is a deep copy of the current instance.
+        /// </summary>
+        /// <returns>The <typeparamref name="T" />.</returns>
+        T DeepClone();
+    }
+}