Menu Customizer: implement basic functionality for input-type-agnostic reordering mode. Items can now be moved up and down and the results are saved via the menu control; sub-items and updating item parents still need to be dealt with.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@945041 b8457f37-d9ea-0310-8a92-e5e31aec5664