Two more events were added to the event type (event.h)
- GPS_TICK -- tick from GPS background task
- GPS_OFF  -- to signal the status bar that the GPS was turned off,
so it can remove the icon

The status bar and GPS coordinate app were changed to react to the
proper events, i.e., both events above in the case of the status bar
and the GPS_TICK event in the case of the GPS coordinate app.