make sure that for a given event (widget/object/module, whatever), only
a *single* input event per button can be registered at one time.
the problem otherwise is with modules that re-register their widgets
with the same IDs (cmus, spotify, etc.): Each time the widget is
re-created (each intervall, typically), it re-registers an input event,
creating an always longer list of callbacks being executed when the
button is clicked (not speaking of the memory leak this introduces).
fixes#668
Add a (half-finished) input library, that for now simply allows
registration and triggering of events.
As next steps, the trigger will happen as part of a separate thread that
reads input events.
Additionally, invoking commands via a execute() will be supported.
Thirdly, there is need of a way to selectively update the affected
modules (widgets), which should be possible given that the event
contains both the instance (widget ID) and name (module name).
