Created FlashPaletteEffectWarhead and removed the hardcoded flashing from NukeLaunch

Not actually hardcoded, but there was a hard dependency in NukeLaunch on FlashPaletteEffect and explicit flashing checks.

OpenRA.Mods.Common/UpdateRules/Rules/20200503/CreateFlashPaletteEffectWarhead.cs

@@ -0,0 +1,61 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2020 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;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace OpenRA.Mods.Common.UpdateRules.Rules
+{
+	public class CreateFlashPaletteEffectWarhead : UpdateRule
+	{
+		public override string Name { get { return "Create FlashPaletteEffectWarhead to replace hardcoded nuke flashing."; } }
+
+		public override string Description
+		{
+			get
+			{
+				return "The trait NukePower (via the NukeLaunch projectile that it uses) no longer has built-in palette flashing.";
+			}
+		}
+
+		readonly List<Tuple<string, string, string>> weaponsToUpdate = new List<Tuple<string, string, string>>();
+
+		public override IEnumerable<string> UpdateActorNode(ModData modData, MiniYamlNode actorNode)
+		{
+			var nukePowerTraits = actorNode.ChildrenMatching("NukePower");
+			foreach (var nukePowerTrait in nukePowerTraits)
+			{
+				var traitName = nukePowerTrait.Key;
+				var weaponNode = nukePowerTrait.ChildrenMatching("MissileWeapon").FirstOrDefault();
+				if (weaponNode == null)
+					continue;
+
+				var weaponName = weaponNode.Value.Value;
+
+				weaponsToUpdate.Add(new Tuple<string, string, string>(weaponName, traitName, "{0} ({1})".F(actorNode.Key, actorNode.Location.Filename)));
+
+				nukePowerTrait.RemoveNodes("FlashType");
+			}
+
+			yield break;
+		}
+
+		public override IEnumerable<string> AfterUpdate(ModData modData)
+		{
+			if (weaponsToUpdate.Any())
+				yield return "Add a FlashPaletteEffectWarhead to the following weapons:\n" +
+					UpdateUtils.FormatMessageList(weaponsToUpdate.Select(x => "Weapon `{0}`, used by trait `{1}` on actor {2}".F(x.Item1, x.Item2, x.Item3)));
+
+			weaponsToUpdate.Clear();
+		}
+	}
+}