In 2023 we still cannot update the MenuFlyoutSubItem.Items(), since #1652 has been around for 4 years

[ruihuasun]

#1652

[ruihuasun]

As the "Hack method" mentioned in #1652, however, if a menu structure is

MenuFlyoutSubItemA
---MenuFlyoutSubItema
------MenuFlyoutSubItem1
----------MenuFlyoutItem1
----------MenuFlyoutItem2
----------MenuFlyoutItem3

Now I want to change items in MenuFlyoutSubItem1, just giving a new MenuFlyoutSubItem instance to MenuFlyoutSubItema is not work. The whole menu item tree should be replaced.