Add EditorPlayer to all mods

To prevent the editor from loading unnecessary or even incompatible
player traits.

OpenRA.Mods.Common/UpdateRules/Rules/AddEditorPlayer.cs

@@ -0,0 +1,40 @@
+#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 System.Linq;
+
+namespace OpenRA.Mods.Common.UpdateRules.Rules
+{
+	public class AddEditorPlayer : UpdateRule
+	{
+		public override string Name { get { return "Add EditorPlayer"; } }
+		public override string Description
+		{
+			get
+			{
+				return "Map editor now requires an EditorPlayer to avoid loading all kinds of unnecessary player traits.";
+			}
+		}
+
+		bool messageDisplayed;
+
+		public override IEnumerable<string> AfterUpdate(ModData modData)
+		{
+			if (!messageDisplayed)
+			{
+				messageDisplayed = true;
+				yield return "The map editor now requires an EditorPlayer actor.\n" +
+					"Please add an EditorPlayer with the traits AlwaysVisible and Shroud to player.yaml\n(or a different rules yaml file of your choice).";
+			}
+		}
+	}
+}