Update documentation
Don't show the pages' link in the menu. We can't set the link as its own parent!
Diverse changes:
Save multiple values for the field as an array.
Add a checkbox if you wish to add the post to the menu (or not!).
Show a select list if the selected menu has no items.
Check the current parent against the DB, not the meta value, as the metadata may be out of date.
More changes:
Add more checks before saving the data.
Check if the menu item exists and just needs to be moved, if it doesn't exist, just create a new link.