Replace IActorInit with an abstract class.

A shared ValueActorInit<T> is introduced to reduce duplication
in the most common init cases, and an ActorInitActorReference
allow actors to be referenced by map.yaml name.

OpenRA.Mods.Common/Traits/Modifiers/FrozenUnderFog.cs

@@ -195,5 +195,5 @@ namespace OpenRA.Mods.Common.Traits
 		}
 	}
 
-	public class HiddenUnderFogInit : IActorInit { }
+	public class HiddenUnderFogInit : RuntimeFlagInit { }
 }