Personal command button like "Borders"

[Pred]

I have my own custom toolbar. I used a mix of menus and submenus. That
works fine.

My new requirement is:
I would like to add a button that would act as a "menu" of buttons like the
Excel "Borders" button.

That means a FaceID with a down arrow on the right side. By clicking on the
down arrow, that would bring a submenu of FaceIDs. Each of the "sub" FaceIDs
would have their own "OnAction".

Any idea on how to do this?