Allow nudging in failsafe Descend mode

[MaEtUgR]

Solved Problem

Being able to nudge the descent in the failsafe mode called Descend using stick input was requested for the following use cases:

• Tethered vehicle
• VTOL with limited hover time
  which are both not allowed to manually be switched to Altitude mode e.g. by RC override

Solution

• Put the horizontal stick mapping of Altitude mode in the StickTiltXY flight task utility class and reused it in Descend mode.
• Unify the blocks for the same stick-mapping strategies. E.g. before the yaw mapping had different filtering and locking logic in Altitude compared to Position mode.

Test coverage

• Simulation/hardware testing logs: Manual testing of the functionality with SITL gazebo iris of the cases losing GPS without RC override enabled and also losing the altitude estimate on top.

[bresch]

Nice

[sfuhrer]

I've tested this new feature on a slightly different base branch, and it worked as expected while not causing noticeable changes to other flight modes. And on the side improves code cleanliness. LGTM!