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).
Add modules to show date, time and date and time. Instead of introducing
a "fancy" aliasing mechanism, simply implement time and date in terms of
datetime, by inheriting and overwriting selectively.
