Support map-defined AI in the lobby.

OpenRA.Game/Map/MapPreview.cs

@@ -210,6 +210,9 @@ namespace OpenRA
 				Status = MapStatus.Unavailable;
 			}
 
+			// Note: multiple threads may try to access the value at the same time
+			// We rely on the thread-safety guarantees given by Lazy<T> to prevent race conitions.
+			// If you're thinking about replacing this, then you must be careful to keep this safe.
 			rules = Exts.Lazy(() =>
 			{
 				try

OpenRA.Game/Widgets/WidgetUtils.cs

@@ -249,8 +249,6 @@ namespace OpenRA.Widgets
 			return trimmed;
 		}
 
-		public static Action Once(Action a) { return () => { if (a != null) { a(); a = null; } }; }
-
 		public static string ChooseInitialMap(string initialUid)
 		{
 			if (string.IsNullOrEmpty(initialUid) || Game.ModData.MapCache[initialUid].Status != MapStatus.Available)