Add menu-item-ancestor class to menu items that have descendants

[valentinas]

As a themes and plugins developer i regularly need to add different styling to menu items that have descendants (for example an arrow or something similar that would indicate that there are submenu items). I have created a plugin for that (​http://wordpress.org/extend/plugins/menu-item-ancestor/), and though it's not very popular, but i honestly believe that this should be a core feature.
Example: ​http://community.mis.temple.edu/ (taken from wp showcase). See how menu items that have submenu items have arrow? So far there's no way to do something like this with CSS in WP. You need to use either plugin or JS.

[ocean90]

See #14041 and #14102.