Rename ProneTime to Duration

More in line with our property naming conventions.

Additionally, added descs to ProneOffset and
ProneSequencePrefix, since at  least the purpose of
the former isn't entirely clear without looking at the code.

OpenRA.Mods.Common/UpdateRules/Rules/20191117/RenameProneTime.cs

@@ -0,0 +1,35 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2019 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
+{
+	class RenameProneTime : UpdateRule
+	{
+		public override string Name { get { return "Renamed ProneTime to Duration"; } }
+		public override string Description
+		{
+			get
+			{
+				return "Renamed TakeCover property ProneTime to Duration.";
+			}
+		}
+
+		public override IEnumerable<string> UpdateActorNode(ModData modData, MiniYamlNode actorNode)
+		{
+			foreach (var takeCover in actorNode.ChildrenMatching("TakeCover"))
+				takeCover.RenameChildrenMatching("ProneTime", "Duration");
+
+			yield break;
+		}
+	}
+}