Fire a `menu-item-added` event after a menu item is added to the DOM. Fire a `menu-removing-item` event before a menu item is removed from the DOM. Enables hooking into and responding to menu changes.
Props welcher, adamsilverstein.
Fixes#31218.
git-svn-id: https://develop.svn.wordpress.org/trunk@39928 602fd350-edb4-49c9-b593-d223f7449a82
