Mission View: Don't show DO_LAND_START as part of mission path.

Currently DO_LAND_START items with locations are displayed as part of the path.

However, the vehicle will never fly to a DO_LAND_START point, the DO_LAND_START location is only used when selecting which DO_LAND_START is uses when there are more than one in the mission. Here the solid yellow path should go directly from waypoint 1 to 3. The locations do matter, so it is important they are still shown, maybe in a similar way to rally points as suggested in https://github.com/ArduPilot/MissionPlanner/issues/2472

Start by locating the Mission View code that renders mission paths and handles DO_LAND_START items, then compare how rally point locations are displayed. Done means the solid path skips DO_LAND_START locations while those locations remain visible for selecting among multiple landing points.

Written by the indexing model from the issue text.