The quantity event handler is not correctly unbound when its action
is deleted, resulting in multiple window.prompt asking the quantity
of the products if you go mulitple times trough a view when a widget
implementing the barcode thing is used.

There seems to be a bug in the framework ("barcode_scanned" events
are registered the same way than the "keypress" one on core.bus
but they don't have suffer from the same issue).