Codechange: Use EnumBitSet for WindowDefaultFlags.

src/engine_gui.cpp

@@ -147,7 +147,7 @@ struct EnginePreviewWindow : Window {
 static WindowDesc _engine_preview_desc(
 	WDP_CENTER, nullptr, 0, 0,
 	WC_ENGINE_PREVIEW, WC_NONE,
-	WDF_CONSTRUCTION,
+	WindowDefaultFlag::Construction,
 	_nested_engine_preview_widgets
 );