Tracker: separate desired angle from target angle

[robertlong13]

This is my first time experimenting with Tracker.

This came about as I was reviewing: https://github.com/ArduPilot/ardupilot/pull/30083

You can test using:

./Tools/autotest/sim_vehicle.py -v ArduPlane -f quadplane --tracker

[wait for load]

arm throttle
mode guided
takeoff 50
[fly to here on map]

[watch the tracker's pitch, nav_pitch, yaw, nav_yaw; I did this using Mission Planner to connect to the tracker's second TCP serial port on 5772]

tracker mode guided
[see nav_bearing and nav_pitch go to 0]

tracker mode auto
tracker position 10 20
[see nav_bearing go to 10 and nav_pitch go to 20]

tracker position 32767 32767
[this is the only way to release the tracker from manual control. This took me forever to figure out; we should make a MAVProxy PR to add a more inuitive way to release control]

tracker mode scan
[see the nav_bearing and nav_pitch follow what scan is telling it to do]