Task for GA fixes

Only for General Aviation: if close to the departure airport consider departure WP already reached and task started, don't display start and finish popups. GA pilots are using the task as their flight plan so first and last WP are the departure and destination airports and they are usually not doing a competition. If we are at departure airport we can consider start line already crossed. And to avoid to display start and and end task popups while beeping during the delicate take off and landing procedures is better for GA.
Other smaller improvements to: LoadGpxTask.cpp and InTurnSector.cpp