handle wgs84 altitude correctly, AMSL also displayed in ft

This PR:
* introduces a new variable ```altitudeWGS84``` for the UAS class. Altitude values from the mavlink message global_position_int are saved into this value while the altitude value from the message VFR_HUD is still saved into ```altitudeAMSL```
* also adds the variable ```AltitudeAMSLFT``` to the UAS class. (AMSL in ft) which is displayed as well in the quick view