[gcs] adapt home icon to zoom level (#679)

not tested yet, I don't have a gps to test with gpsd2ivy
[gcs] change GCS icon size on zoom change event
[build] build in var/aircrafts/<ac_name> instead of var/<ac_name>

closes #649
update changelog
[radio] SBus Radio
[radio_control][sbus] Dual receiver setup
[radio_control][sbus_dual] Remove double code
update changelog
[sim] fix sim launcher
[modules] nav_drop: more configurable

should close #695
fix path to var/aircrafts/<ac_name> in various tools

especially path to flight_plan.xml and settings.xml files copied to var/aircrafts/<ac_name>
[modules] nav_survey_poly_osam: remove unused functions
[fixedwing] add NavSetAltitudeReferenceHere
update changelog
[ardrone_vision] Vision Submodule
[guidance] fix IGAIN precision and add VGAIN

based on #682

this may introduce too large horizontal guidance IGAIN in rotorcraft
airframe files
[ahrs] settable heading deviation threshold cmpl_quat filter
[ahrs] int_cmpl_quat: configurable AHRS_HEADING_UPDATE_GPS_MIN_SPEED

Only used if AHRS_USE_GPS_HEADING is true.
Don't update heading from GPS course if GPS ground speed is below is this threshold.
Defaults to 5.0m/s which is the same as hardcoded value before.
[conf] lower GUIDANCE_H IGAIN on some quadrotor airframes
[nps] sensors param for ardrone2