I also took the freedom to add type checking, as we can't limit the
usage of `Add()` since it's a Container. The exception thrown also
advises of using the suggested `AddTrigger()` instead.
Functionally equivalent right now, but the combined variant is more
localised to what it actually needs to do, and less error-prone if
any new code gets appended to the method.
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.