FixedWingFollower: sync with ManualControlCommand

The manual control module no longer sets PathDesired, and this
is the responsibility of the fixed wing follower. This updates
it accordingly.
Flight/FW Follower: make sure to process updated path desired
Flight/FW Follower: do not update PathDesired out of sync with main loop

This is a potential race condition
Flight/FW Follower: use a clearer state variable

Make it explict what the values of the follower state variable
are rather than using a boolean.
Merge pull request #1350 from peabody124/fixedwing_follower

FixedWingFollower: sync with ManualControlCommand