App messages now use simple modal message dialogs. Vehicle messages continue to use non-modal popup.

Support up to 64 buttons, not limit to 16
Send a single button command and not a stream of commands at 25Hz
Send button commands through signals (it's coming from a different thread)
Hide gimbal support through analog channels (there are no MAVLink messages to handle it)

Add gimbal to external control signal

Add camera trigger
Add video start/stop/toggle recording
Add gimbal pitch and yaw

Joystick axes and buttons can now be monitored even when the
joystick input is disabled.

Minor improvement to detection of stick recentering after calibration. Without this if you performed a calibration with deadband enabled but one of the deadbands at the end of the calibration was ~0 it would be very difficult to position the stick so that Qgroundcontrol considered the axis to be centered.

recentered after calibration.

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