To implement different different sources of input for KeyCounter, it
is now possible to create a Trigger class (to inherit) instead of
inheriting KeyCounter. This eases the creation of more input sources
(like for tests) while allowing to implement different UI variants.
That way, if another variant of the key counter needs to implemented
(for whathever reason), this can be done by only inheriting KeyCounter
and changing how things are arranged visually.
