Change: Add window description flag to disallow interactive window closing. (#11008)

This replaces/simplifies testing for a closebox to allow closing a window with right-click, and testing for specific window classes when closing all windows by hotkey.

This allows right-click closing of dropdowns and the high-score window.

src/main_gui.cpp

@@ -510,7 +510,7 @@ struct MainWindow : Window
 static WindowDesc _main_window_desc(
 	WDP_MANUAL, nullptr, 0, 0,
 	WC_MAIN_WINDOW, WC_NONE,
-	0,
+	WDF_NO_CLOSE,
 	_nested_main_window_widgets, lengthof(_nested_main_window_widgets),
 	&MainWindow::hotkeys
 );