Some GPS variables not set by the stock sim

The GPS variables GPS APPROACH MODE and GPS APPROACH IS FINAL are not set by
the stock sim while they are documented so we can’t use them. Working Title
flight planner sets them correctly in their flight planner module when using
the Nxi.

Hello @pms50 I suspect it’s related to the issue described here:
https://devsupport.flightsimulator.com/t/4014 Basically, the flight plan
manager doesn’t switch to approach on its own and expects some input from the
avionics. I had no feedback on the solution I suggested so let me know if this
is relevant. Regards, Sylvain