Add class name to first menu item in wp_nav_menu()

[johnbillion]

If the theme displays the output of wp_nav_menu() horizontally and wants to put a separator between menu items, then usually something like a left border is used.

To remove this separator from the left hand side of the menu, the first item will often have the border or background image removed by using the ':first-child' CSS pseudo class. This is all well and good but the ':first-child' selector isn't supported in IE6.

If we added a 'first-child' or 'first-menu-item' class to the first menu item then it'd be easier for themers to use this property and maintain support for IE6.

Thoughts?

[johnbillion]

Patch. I think this covers it.

[nacin]

Duplicate of #14250 and at least one other, I think. We don't wish to do this. There's code in that ticket that can do this on a filter.