Bug 6448 [3/3] EAN-13 barcode labels

Bug 6448 [3/3] EAN-13 barcode labels

Current implementation doesn't scale barcodes because low-price
CCD barcode readers are very sensitive about size

Test scenario:

1. in Tools > Labels create or edit Layout and select EAN13 as barcode
   type

2. export one of existing batches using EAN13 layout and verify that
   generated pdf file contains barcodes

3. print pdf file and test it with barcode reader

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>