Data: show target position (not just direction)

If a Plane is flown in Guided mode (and perhaps other vehicles in other modes), on the Data screen the orange line shows the direction to the target position but not the distance. It would be easier for users if the actual target was displayed (distance+length). Below is a comparison of what MAVProxy shows vs MP

FYI @tridge

Start at Mission Planner's Data screen and reproduce the issue with a Plane in Guided mode, using the supplied MAVProxy-versus-MP comparison as the reference. Trace where the orange target-direction line is drawn and determine how target distance is available; done means the target is displayed with both direction and distance, including its visible length.

Written by the indexing model from the issue text.