Adds menu icons, menu item css class, link XFN, menu item description, ability to target link to new tab.
Most themes use custom menu walkers, instead of wp default Walker_Nav_Menu. And some of developers don't call 'walker_nav_menu_start_el' filter. You need to find menu walker class in your theme code (search for files which contain 'extends Walker_Nav_Menu' and 'extends Walker') and add apply_filter rule at the end. See wp-includes/nav-menu-template.php as example. Alternatively you can use get_post_meta($menu_item_id, '_vihv_menu_icon', true) to receive icon url inside menu walker code and use it as you want.
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