Menu Customizer: adjustments required to make dynamic section-adding for new-menu-adding functional. Much of this will end up in WordPress core eventually, see #core-30741.

Fixes #2335.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@1046535 b8457f37-d9ea-0310-8a92-e5e31aec5664