Bug 7468: Print barcodes by range

Bug 7468: Print barcodes by range

Adds an option to the label creator to print a range of barcodes.
Only allows printing to PDF.

C4/Label.pm had to get minor changes to override database-based barcode
generation. Same with labels/label-create-pdf.pl. By default, the
barcode is fetched from the database using the itemnumber, but when
printing ranges, you might not have a corresponding  database entry.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>