Menu Customizer: move all descendants of a menu item with it when moving an item into or out of a submenu. Still to do: move all descendants with an item when moving it with the reorder buttons. Requires updating item's parent id when the parent is cloned (as its id changes), which in turn triggers clones of all descendant items. Possible alternative would be mapping original items to the cloned ones when finding parents in PHP/output, but this works pretty well for now.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@967925 b8457f37-d9ea-0310-8a92-e5e31aec5664