Add approaching-(short-)runway when aircraft is airborne

* Added approaching-runway and approaching-short-runway when aircraft is airborne
    * Distance to runway < 3 nm
    * Altitude above runway must be 300 - 450 ft or 550 - 750 ft
    * Alignment with runway < 20 degrees
    * takeoff_config.nominal_distance_landing_m is by default 2000 meter. You could tune it depending on weight, runway condition dry/wet and airport elevation. See last 2 slides of chapter 3 of 747-8i on http://www.boeing.com/boeing/commercial/airports/747.page
* Defined a \<logic\> element in Systems/747-8-autopilot.xml to set /gear/on-ground so that there is only one listener instead of two. You can move it to another file or rename the property if you want.