This reverts commit 7131a12e.

The lines removed set the center point of the throttle axis to one extreme of the joystick range. This causes an offset value when the stick is centered if the joystick does not return exactly zero. This is important for vehicles with reversible throttle, such as ground robots or boats.

Joystick: Fix calibration and default use: Do not send anything to vehicle until enabled. Enable after calibration by default.

Instead us a Toolbox concept which hangs off of QGCApplication

When on Joystick config page:
- Not calibrating: If joystick is enabled, joystick messages should
continue to go to vehicle