Add 'menu-item-has-children' class to parent items like in nav-menu-template.php

Add 'menu-item-has-children' class to menu items with children to match the original behavior in wp-includes/nav-menu-template.php so we don't lose any functionality when using as a drop-in-replacement.

Not sure if these classes were left out intentionally, but I don't consider this as a huge performance hit.

Thanks for the great plugin!