Menu Customizer: remove the position parameter for menu item controls - controls don't need to know their own position in JS (order is managed by the menu control). Add an original_id parameter that matches the menu_item_id in PHP but will be used to determine if an item is cloned in JS.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@945748 b8457f37-d9ea-0310-8a92-e5e31aec5664