Allow plugin and theme authors to add extra fields to the navigation menu items editor

[sayontan]

If a plugin / theme author has to add input fields to the navigation menu editor, the only way is to extend the Walker_Nav_Menu_Edit class, replicate the start_el function, and modify this function to add fields. This can cause issues upon core upgrades. I am attaching a patch that includes an action hook after all the core fields are shown, so that plugin / theme authors can use this hook to add more custom fields. A corresponding hook for saving is not needed, since wp_update_nav_menu_item takes care of that.

[ocean90]

#14414, #18584