Made it possible to extend level3d prefab (#80)

hide/view/l3d/Level3D.hx

@@ -282,7 +282,7 @@ class Level3D extends FileView {
 	function get_properties() return sceneEditor.properties;
 
 	override function onDisplay() {
-		data = new hrt.prefab.l3d.Level3D();
+		data = cast(hrt.prefab.Library.create("l3d"), hrt.prefab.l3d.Level3D);
 		var content = sys.io.File.getContent(getPath());
 		data.loadData(haxe.Json.parse(content));
 		currentSign = haxe.crypto.Md5.encode(content);