Menu Customizer: add newly added menus as options in the theme locations dropdowns. Note that while this all works, and you can now create, build, and set a menu in the Customizer, you currently have to save and then trigger a preview refresh before items added to a newly added and set menu will preview.

See #2337.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@1047651 b8457f37-d9ea-0310-8a92-e5e31aec5664