Add ability to set and sort post children, using post_parent and menu_order

In Fieldmanager, you can currently set a post's parent using the `post_parent` field, but you can't use that field to set the relationship from the parent's editing screen. It would be a cool feature to be able to set post children, and even leverage the `menu_order` field when sorting.

There's an important complication to consider, which is that a post can only have one `post_parent` and one `menu_order`. We'll need to consider this case, and handle situations where one or the other is already set.