If the module barcodes is installed, a BarcodeEvents singleton is
automatically instanciated. It listens to keypresses and, when a
sequence of keys represents a barcode, it broadcasts a 'barcode_scanned'
event on core.bus containing the barcode string.

Let arrow keys, tab and escape pass through. We still want users to be
able to use eg. arrow keys to be used when selecting a radio item. This
will always work in Chrom{e,ium} because they do not dispatch keypress
events for these types of keys, only keydown/keyup events. Firefox does
dispatch keypress events. This makes sure that we do not touch those
types of events in Firefox.

devices have to fire keypresses every 50ms minimum

Previously this was 100ms. This works just as well, but was mainly in
there for testing purposes.

Before this, users were only able to press a key when all other keys
were released to prevent a key from repeating. This works but when
typing you usually press the next key before releasing the current
one. This allows users to do exactly that while still blocking keys from
repeating.

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.