Add power shutoff warning to GNC255

[tigert]

I am just collecting the features of the GNC255 here.

This happens when you switch the power to OFF by twisting the COM volume knob to min (the potentiometer has a detent) - the unit shows a warning of immediate shutdown - maybe we can just add a Message type that shows this over everything else when it is TRUE, and do the rest of the logic in Connector.