Changed Joystick._exponential from bool to float. It can now be set between 0 and -0.5 by user using a slider.

Instead us a Toolbox concept which hangs off of QGCApplication

- select active vehicle
- arm/disarm
- select flight mode

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

Accidentally deleted somehow