Fix crash in Beacon ctor when it runs during instead of between ticks

Causes an exception like below, for example when calling the Lua API's Beacon.New() method.

``Exception of type `System.InvalidOperationException`: Collection was modified; enumeration operation may not execute.``

OpenRA.Mods.Common/Effects/Beacon.cs

@@ -55,7 +55,7 @@ namespace OpenRA.Mods.Common.Effects
 			}
 
 			if (duration > 0)
-				owner.World.Add(new DelayedAction(duration, () => owner.World.Remove(this)));
+				owner.World.AddFrameEndTask(w => w.Add(new DelayedAction(duration, () => owner.World.Remove(this))));
 		}
 
 		// Support power beacons are expected to clean themselves up