Multiple barcodes on products

This module adds a barcode list to products so that one item can be identified by more than one barcode.

Where it appears: - On the product form and the product variant form, a new editable "Barcodes" list is shown right after the barcode field, with a drag handle to set the order. - On the product template form the list is only shown when the template has a single variant. - In the product search view, a "Barcodes" field is added so products can be searched by barcode.

Business rules and automatic behaviour: - The standard barcode field becomes read-only and is computed from the first barcode in the list (ordered by sequence). Editing it through other means writes the value back into the barcode list. - A barcode cannot be used twice: saving a duplicate raises an error naming the product and company that already use it. - Each barcode record is linked to a product and its template, and the company is taken from the product. - When the module is installed, existing single barcodes on products are copied into the new barcode list.

It touches the product, product variant and product template areas, plus a new barcode record type, and all internal users can read, create, edit and delete barcode entries.