Add GTIN property or function field
In the selection of the product identifier type there is the key 'ean' which has a string of "International Article Number".
The right string for the key 'ean' would be "European Article Number (EAN)" but it seems that even this is not a good idea as this standard has been integrated into the Global Trade Item Number (GITN).
I propose to replace the tuple ('ean', 'International Article Number') by ('gtin', 'Global Trade Item Number (GITN/EAN/IAN)').
I think this should be done before the release.