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/Render/WithWallSpriteBody.cs

@@ -168,13 +168,9 @@ namespace OpenRA.Mods.Common.Traits.Render
 		}
 	}
 
-	public class RuntimeNeighbourInit : IActorInit<Dictionary<CPos, string[]>>, ISuppressInitExport
+	public class RuntimeNeighbourInit : ValueActorInit<Dictionary<CPos, string[]>>, ISuppressInitExport
 	{
-		[FieldFromYamlKey]
-		readonly Dictionary<CPos, string[]> value = null;
-
-		public RuntimeNeighbourInit() { }
-		public RuntimeNeighbourInit(Dictionary<CPos, string[]> init) { value = init; }
-		public Dictionary<CPos, string[]> Value { get { return value; } }
+		public RuntimeNeighbourInit(Dictionary<CPos, string[]> value)
+			: base(value) { }
 	}
 }