[IMP] barcode_events: implement a new way of dealing with barcode events

Implement a new way of dealing with barcode events. This was necessary
because the old way of dealing with input coming from barcode devices
did not stop keyevents. This lead to all kinds of issues (eg. barcode
reader output being inserted as tender in a paymentline).

This new class inserts itself so that it handles all keyevents before
everything else. This means that it has authority on what keypress
events pass and which ones do not. It differentiates between 'real'
keyboard input and input coming from devices by buffering events. When a
certain amount of events come in fast enough, it is handled as a barcode
and a 'barcode_scanned' event is emitted on core.bus. When buffered
events are decided to not be a barcode, they are recreated and
redispatched.