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).
