#33 Driver elements with feedback

[G.A. Cignoni]

It will be nice to have interface elements for the
Driver which get feedback from the system.

The interface element is connected with a pair of
external and exported attributes, the exported returns
the feedback to the interface element.

For instance a switch with a led, the led status is
controlled by the system and also controls the input to
the external.

Alternative: a button which sends a true for just a cycle.