Visualise attitude and depth targets on widgets

[ES-Alexander]

This might be a pain to extract from the autopilot, but it could be valuable to the operator to see the current attitude and depth targets on the relevant widgets (compass, pitch indicator, altitude indicator, etc), similar to how it's valuable to visualise the planned path and waypoints on the map while executing a mission.

It may also be nice to allow setting one or several custom (frontend-only) indicators, either for controlling the vehicle in manual mode, or just as a general way of visualising nearby points or objects of interest.

Consider visualising landmarks in the surroundings (opacity indicates proximity):

Or detected obstacles, or known positions of other vehicles within a fleet: