Menu Customizer: adda UI for deleting menus. This is accessed through a deletion-mode-toggle in the new menu section, which exposes delete buttons for each menu. Users are prompted with an AYS before the deletion request is submitted. Server-side deletion processing to come.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@944082 b8457f37-d9ea-0310-8a92-e5e31aec5664