Copter: Better RTL travel altitude

Tip fo better RTL travel altitude.
Current bahaviour of RTL is: if altitude lower then altitude for RTL, then ascend and return to home location, then descent vertically. If altitude is higher, then travel home at current altitude and descent.

Better option would be: if alt. higher, then travel home and gradually descent to RTL altitude (based on max descent speed), then over home location descent vertically. This will speed up time for returning to land and during forward movement, quad can descent faster then in zero speed (vortex ring). There is less chance then quad will drain battery before it lands.

The issue describes Copter RTL travel and descent behavior when the vehicle is above the RTL altitude. Start by locating the Copter RTL implementation and related altitude or flight-behavior tests, then compare the current behavior with the proposed gradual descent. Done means the vehicle descends toward RTL altitude while traveling home and performs the final vertical descent over home.

Written by the indexing model from the issue text.