Forward links should forward HEARTBEAT message too

For follow-me, ship landing project and other applications it is useful to be able to forward one vehicle's location to another. MP's has such a feature (I've just discovered today) using Data, right-mouse-button-click, forward between links.

The issue though is that this only sends the ATTITUDE and GLOBAL_POSITION_INT messages. I think we should also be sending the HEARTBEAT so that readers of the mavlink messages know what type of vehicle they are dealing with.

Here is a screenshot of a test with a quadcopter (vehicle1) and a shiplanding beacon (Vehicle 10). We can see that for vehicle 10 we only get the ATTITUDE and GLOBAL_POSITION_INT messages

@tridge, I think you were the original requestor of this feature so just FYI

No file or test is named. Start at Mission Planner's Data → forward between links behavior; done means a forwarded vehicle also emits HEARTBEAT alongside ATTITUDE and GLOBAL_POSITION_INT.

Written by the indexing model from the issue text.