product_state: allow to set state per product variant

[diggy128]

Is your feature request related to a problem?
You cannot set a state for a variant without affecting the template product (and thus all variants).
Let's say you have a template product eg T-shirt with 4 different variants by color (black, white, red, blue).
Let's assume that the blue ones are so low on sales that you decide to stop selling that variant (while keeping the rest).
It should be possible to set state only for the blue one as obsolete while keeping the others as sellable.

Describe the solution you'd like
Disconnect (or even drop) state from product.template.
Product count should be on product variants to reflect the number of variants in that state (since it's the variants that affect the stock available).