added selection-changed hook

OpenRa.Game/Selection.cs

@@ -25,6 +25,9 @@ namespace OpenRa
 
 			var voicedUnit = actors.FirstOrDefault(a => a.traits.Contains<Unit>() && a.Owner == world.LocalPlayer);
 			Sound.PlayVoice("Select", voicedUnit);
+
+			foreach (var ns in world.WorldActor.traits.WithInterface<INotifySelection>())
+				ns.SelectionChanged();
 		}
 
 		public IEnumerable<Actor> Actors { get { return actors; } }

OpenRa.Game/Traits/TraitsInterfaces.cs

@@ -99,5 +99,7 @@ namespace OpenRa.Traits
 		public object Create(Actor self) { return Instance.Value; }
 	}
 
-	interface ITraitPrerequisite<T> { }
+	public interface ITraitPrerequisite<T> { }
+
+	public interface INotifySelection { void SelectionChanged(); }
 }