Codechange: make WindowClass a scoped enum

src/help_gui.cpp

@@ -205,7 +205,7 @@ static constexpr std::initializer_list<NWidgetPart> _nested_helpwin_widgets = {
 /** Window definition for the help window. */
 static WindowDesc _helpwin_desc(
 	WindowPosition::Center, {}, 0, 0,
-	WC_HELPWIN, WC_NONE,
+	WindowClass::Help, WindowClass::None,
 	{},
 	_nested_helpwin_widgets
 );