For a model to use it, it must :
- Inherit barcodes.barcode_events_mixin and override on_barcode_scanned(self, barcode)
- Add <field name=barcode_scanned widget=barcode_handler/> to each form view that
should listen to barcode events and trigger on_barcode_scanned.
The method on_barcode_scanned works just like an onchange.
