Fields values for order items are not stored

Problem/Motivation

Commerce core provides the ability to add fields to order item types. That enables us to store additional information on a per order item basis. Those fields are added to the "add to cart form" also provided by commerce.

commerce_wishlist adds an "add to wishlist" button to the "add to cart" form. If the user enters values into those fields and adds the product variation to the wishlist, those values are not stored. Even worse: there is no way to modify those values after that.

Steps to reproduce

1. Add a text field to the order item type being used and make it required
2. Add a value to the field in the add to cart form and click the "add to wishlist" button
3. Go to the wishlist and add the product to the cart
4. Go to the cart
5. There is no value set for the field

Proposed resolution

We should find a way to store values of order item fields and set them if a wishlist item is added to the cart.

If possible, modifying those values would be nice as well.

Remaining tasks

User interface changes

API changes

Data model changes