Implement Nav OBS (VOR radial) display for GNC255

[tigert]

Another thing to note down for future reference on how to improve the gnc255 display

Pressing the OBS button toggles the VOR radial view on the lower part of the screen:

Shows selected radial for the tuned NAV frequency,

• Add OBS mode message type
• Add OBS radial (CRS) message type
• Display graphical OBS indicator that shows:
  • station id (this might need to be added too as a data type)
  • set VOR radial (OBS/CRS) for the course display
  • show the course display as inverted (another data type,
    used to indicate its adjustable with dual encoder)
  • CDI deflection scale (I guess it needs a data type for the
    numeric value from -10 to 10 for the deflection)
  • show "CM ACT" in small and the active COM frequency as a reference