Mavlink terrain protocol support

Remove some more Follow stuff

Remove ArduPilot Follow Me support from Stable

Drive Plan view ROI support from firmware capability bits

Cleanup download object on error

Change follow to work above home altitude only

This adds support for the new ArduSub 3.0 feature of automatically detecting
the thrusters directions and reverting the corresponding motor output if
necessary

Allow plugin to add flight modes to joystick button actions