Menu: must update items on modifying the list while showing

 Issue: modifying the items of a Menu (aka: submenu) while it is showing
 does not update the items shown. For comparison: modifying the list of
 direct items of the contextMenu is working as expected.

 To reproduce, compile the example below, run:
 A: see tab "subMenu"
 - right click in button the show contextmenu
 - move mouse over menu to open submenu and make sure it remains open during the steps below
 - press f1
 - expected: first item of open submenu visibly removed
 - actual: no visual change
 - to verify that items are removed from the items list, press f1 until message "empty" is printed

 for comparison, see tab "direct" and do the same:
 - as expected, items are visibly removed from the showing popup
 
The issue turned up in a question at SO: https://stackoverflow.com/q/54834206/203657

The culprit seems to be ContextMenuContent that is responsible for opening/hiding submenus - it does so by calling:

    private void showSubmenu(Menu menu) {
        openSubmenu = menu;
        createSubmenu();
        submenu.getItems().setAll(menu.getItems());
        submenu.show(selectedBackground, Side.RIGHT, 0, 0);
    }

note that the items of the subMenu (== ContextMenu that's showing the items of the menu) is set once to the items of the menu, without registering any listeners: so removing the items from Menu's items doesn't keep the items of the subMenu in sync.

A dirty (! and beware: not tested for side-effects) hack around is to

- grab the ContextMenuContent for the menu (that is the content that contains menu)
- reflectively access its private field subMenu
- when changing the items in menu, reset the items in subMenu to the same

public class MenuModifyItemsWhileShowing extends Application {

    protected Node createTabContent(boolean asSubMenu) {
        final List<String> options = Arrays.asList(
                "AbC",
                "dfjksdljf",
                "skdlfj",
                "stackoverflow");

        ContextMenu cmenu = new ContextMenu();
        List<MenuItem> items;;
        if (asSubMenu) {
            //---------- submenu: bug
            final Menu menu = new Menu("MENU");
            cmenu.getItems().setAll(menu);
            items = menu.getItems();
        } else {
            // for comparison: directly added - behaves as expected
            items = cmenu.getItems();
        }
        options.stream().map(MenuItem::new).forEach(items::add);

        Button button = new Button("for contextMenu");
        button.setContextMenu(cmenu);
        // on pressing f1, remove first menuItem until list empty
        cmenu.addEventFilter(KeyEvent.KEY_PRESSED, e -> {
            if (e.getCode() == KeyCode.F1) {
                if (!items.isEmpty()) {
                    items.remove(0);
                } else {
                    System.out.println("no more items");
                }
            }
        });
        return button;
    }

    private Parent createContent() {
        TabPane pane = new TabPane();
        pane.getTabs().addAll(
                new Tab("subMenu", createTabContent(true)),
                new Tab("direct", createTabContent(false)));
        return pane;
    }

    @Override
    public void start(Stage stage) throws Exception {
        stage.setScene(new Scene(createContent()));
        stage.setTitle(FXUtils.version());
        stage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
    
}