Prevent adding child products to products with stock records

The implicit conversion of a standalone product to a child product only
works if certain conditions are met. As far as I can tell, the only
condition preventing a conversion would be when the product already has
stock records -- parent products aren't allowed to have any.
Don't show child products in product list view

I was under the impression that's already the case. But especially with
the redesign of how child products are edited, it makes more sense to
only show parent and standalone products.
Pass validation and improve workflow

Variant products can now be created from a suitable future parent
product. A helpful message is displayed if the future parent is not
suitable.
Two new paths have been added to handle adding a variant from a suitable
parent, and for adding another variant after saving one.

Some more tweaking was required to pass the tight product validation.