Reinstate 'label' property of nav menu items

[johnbillion]

Using the nav_menu_items_{$post_type} filter, a plugin can add to the list of nav menu items available on the Appearance -> Menus screen.

Prior to [22234] a nav menu item could include a 'label' property and this would be used as the item title when shown in the list, allowing the actual menu item to use a different title. That changeset removed the use of the label property.

One of my plugins uses the 'label' property for this purpose. It'd be nice to reinstate it please and thank you.

[nacin]

Looks good. Marking for 3.6 as this was a 3.4 -> 3.5 regression.

[markjaquith]

In 24798:

Reinstate 'label' property of nav menu items. Fixes a regression from [22234].

Fixes #24840. Props johnbillion.