Menu Customizer: first pass at scalable menu-saving functionality. Menu item addition, deletion, and reordering (partially) should now be saved when the "Save and Publish" button is pushed, although these are not yet preview-able. Next steps are to implement cloning and saving/deleting of nav menu items when their fields change, to prevent menu item data from being posted when saving the customizer, and to continue cleaning up both the UI and the implementation.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@944650 b8457f37-d9ea-0310-8a92-e5e31aec5664