Add ISingleInstanceInit interface.

Inits that are logically singletons (e.g. actor
location or owner) should implement this interface
to avoid runtime inconsistencies.

Duplicate instances are rejected at init-time,
allowing simpler queries when they are used.

OpenRA.Game/Primitives/TypeDictionary.cs

@@ -41,6 +41,11 @@ namespace OpenRA.Primitives
 			return data.ContainsKey(typeof(T));
 		}
 
+		public bool Contains(Type t)
+		{
+			return data.ContainsKey(t);
+		}
+
 		public T Get<T>()
 		{
 			return (T)Get(typeof(T), true);