Add measurement directions for remote sensing overlay

[Joern Ungermann]

To properly align expected structures with the remote sensing instrument, it would be advantageous to provide markers indicating the currently configured measurement direction.

The envisioned patch shall add a solid line to the tangent point overlay that indicates the viewing direction. By modifying the viewing angle that line can be aligned with the atmospheric structures.

[Reimar Bauer]

Added indicator for measurement direction for legs in remote sensing widget.

Fix issue #294

→ <<cset 47b9453249c6>>

[Reimar Bauer]

Remove arrow for remotesensing direction indicator for very small legs.

For actual flightplans for ATC some very short legs were introduced that generated, strangely, artefacts in the graphical representation of the viewing direction.

See issue #294

→ <<cset 5893a11ccb67>>