Add Replaceable and Replacement traits to support wall overrides.

OpenRA.Mods.Common/Traits/Replacement.cs

@@ -0,0 +1,25 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2018 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using System.Collections.Generic;
+using OpenRA.Traits;
+
+namespace OpenRA.Mods.Common.Traits
+{
+	public class ReplacementInfo : TraitInfo<Replacement>
+	{
+		[FieldLoader.Require]
+		[Desc("Replacement type (matched against Conditions in Replaceable).")]
+		public readonly HashSet<string> ReplaceableTypes = new HashSet<string>();
+	}
+
+	public class Replacement { }
+}