showing another GCS in vehicle list

If there are multiple GCS on the radio link with different sysid then MissionPlanner can end up showing the other GCS in the vehicle list:
in this example sysid 251 is a mavproxy instance that was forwarding to MissionPlanner
This led to some confusion as MissionPlanner defaulted to the mavproxy GCS as a vehicle, and showed the copter flight modes for a quadplane.
We should be able to exclude GCS instances using HEARTBEAT MAV_TYPE of GCS

The issue identifies Mission Planner’s vehicle list and HEARTBEAT MAV_TYPE as the relevant entry points. Start by tracing how HEARTBEAT messages and sysids populate the list; done means GCS instances such as MAVProxy are excluded, so Mission Planner does not treat them as vehicles or show their flight modes.

Written by the indexing model from the issue text.