Menu Customizer: add a function that renders the html to add a new menu. This includes the markup for the section and all of its default controls. This will be necessary until https://core.trac.wordpress.org/ticket/28709 happens.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@942506 b8457f37-d9ea-0310-8a92-e5e31aec5664
Menu Customizer: first-pass ajax handler for adding new menus.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@942507 b8457f37-d9ea-0310-8a92-e5e31aec5664