Menu Customizer: use the new Customizer JS API in WordPress 4.1 to dynamically add new menus as Customizer sections. Nav_menu controls are added to new menu sections, theoretically making it possible to add menu items to the menu without a pageload as well. In the future, the other controls needed for an empty menu will also be added dynamically here, but they both use the base control, which doesn't yet support being rendered from JS. Note that the section UI is not yet rendered, meaning that the controls added to it can't be rendered either.

See #2335

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@1045819 b8457f37-d9ea-0310-8a92-e5e31aec5664