Change menu found at PATH as item NAME to contain ITEMS.
PATH is a list of strings for locating the menu that
should contain a submenu named NAME.
ITEMS is a list of menu items, as in `easy-menu-define'.
These items entirely replace the previous items in that submenu.
If MAP is specified, it should normally be a keymap; nil stands for the local
menu-bar keymap. It can also be a symbol, which has earlier been used as the
first argument in a call to `easy-menu-define', or the value of such a symbol.
If the menu located by PATH has no submenu named NAME, add one.
If the optional argument BEFORE is present, add it just before
the submenu named BEFORE, otherwise add it at the end of the menu.
To implement dynamic menus, either call this from `menu-bar-update-hook' or use a menu filter.