Nav_Menus component is not ready to handle many nav menus

[benoitchantre]

Issue Overview

The component seems limited to handle only one nav menu. I think it should ready to handle multiple nav menus, because a theme often has many nav menus.

WP Rig should be usable with many nav menus without having to refactor the methods of this component.

Current Behavior

We have a PRIMARY_NAV_MENU_SLUG string to store nav menu slugs. This constant limits the usage of the component to one nav menu without having to refactor it.

Possible Solution

Rename PRIMARY_NAV_MENU_SLUG to $nav_menus. Add getters and setters to get/set the nav menus as an associative array with menu slugs and descriptif texts.

Refactor methods to use that array.