Menu Customizer: first pass at menu item depth management, via the reorder buttons. Needs work around reordering with sub-items, but semi-functional for now.

Introduce (commented-out) logic for drag-and-drop submenus from nav-menu.js.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@967716 b8457f37-d9ea-0310-8a92-e5e31aec5664