Define and measure duration for text notifications in milliseconds

During a game notification duration should be the same regardless of
game speed. Switch to using wall-clock time defined in milliseconds 
instead of game ticks. Also use the opportunity to rename the field 
to "Duration" because "RemoveTime" is not so clear.

OpenRA.Mods.Common/UpdateRules/Rules/20221203/TextNotificationsDisplayWidgetRemoveTime.cs

@@ -0,0 +1,39 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2022 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;
+
+namespace OpenRA.Mods.Common.UpdateRules.Rules
+{
+	public class TextNotificationsDisplayWidgetRemoveTime : UpdateRule
+	{
+		public override string Name => "Change name and unit of RemoveTime field of TextNotificationsDisplayWidget.";
+
+		public override string Description =>
+			"Change the field name from RemoveTime to DisplayDurationMs and convert the value from ticks to milliseconds";
+
+		public override IEnumerable<string> UpdateChromeNode(ModData modData, MiniYamlNode chromeNode)
+		{
+			if (!chromeNode.KeyMatches("TextNotificationsDisplay"))
+				yield break;
+
+			foreach (var field in chromeNode.ChildrenMatching("RemoveTime"))
+			{
+				field.RenameKey("DisplayDurationMs");
+
+				var durationMilliseconds = field.NodeValue<int>() * 40;
+				field.ReplaceValue(FieldSaver.FormatValue(durationMilliseconds));
+			}
+
+			yield break;
+		}
+	}
+}