flightgear hil: correct scaling of pressure for mavlink message and remove indicated airspeed from flightgear protocol, commit requires flightgear protocol update

Replace 9 seperate valueChanged() signals with just 1, and remove the resultant tons of code duplication

* The "Simulation" option is now not selectable.
* If the link is a simulation, then you can't change it to a different link type.
* The OPAL option is not shown if support is not compiled in.
* Fix the buddy, tab order and accelerators in the GUI.

This crash could be reproduced by starting QGroundControl, switching to
plot, then clicking the "Symmetric" checkbox.

This commit disabled RC RAW signal callback.