Menu item parent property

[filosofo]

Currently, the hierarchy of menu items works haphazardly: a menu item's parent is determined by its post_parent property, which is the database ID of the item's original object.

This introduces a number of bugs because menu item original objects can be either taxonomy objects or post objects, each group of which has its own independent set of database IDs. So try to order a mixture of taxonomy menu items and post type menu items, and you get inconsistent (sometimes non-functional) results.

Patch introduces the menu item property menu_item_parent, which determines the hierarchical placement of the given menu item. menu_item_parent refers to the menu item DB ID of its parent, if any, which is internally consistent because all menu item objects are post objects.

[duck_]

Related: #13037

[nacin]

(In [14450]) Non-JS menu ordering fixes, also parent handling fixes. props filosofo, fixes #13135, fixes #13249

[dickie]

I'm not sure if this is the fix that broke it, up to the point that this was checked in the menu were working OK, but it is now not possible to save menu items as children. You can physically move them, but once saved the hierarchy is lost.
(Note this is with Javascript ENABLED.)

[filosofo]

dickie, please see #13256 and re-up.