Add "Barcode/Product condition" to Promotions

This feature would be useful for both vouchers and promotions like "Buy Item-A get 50% off Item-B". The current system can't really do this as buying 2 * Item-B would also trigger the discount.

So I would like to either specify barcode(s) that "trigger" the discount as a condition, or select a single product as the condition. (I could then setup vouchers to be products with a price of £0.00).

It should be possible to combine this with the "Quantity condition" so that we can set 1 voucher per product discount for example.

A current workaround is creating a "Voucher" payment type, but this requires using split payment and manually calculating the value of the voucher, which is slow. Also this workaround doesn't solve the problem of doing "Buy Item-A get 50% off Item-B" promotions.
Allowing negative priced products could also work for vouchers (and Lottery payouts), but I understand that may be a complicated change.

Thanks for the awesome software, and thanks for making it free!