Add Blimp icon support

AP supports blimps now but it seems that MP shows them as multicopters on the map. It would be nice if we had a blimp icon appear instead. [The MAV_TYPE for Blimps (aka Airships) is 7](https://github.com/ArduPilot/mavlink/blob/master/message_definitions/v1.0/minimal.xml#L94).

In addition I'm working on adding support for reducing the oscillation of a slung payload ([see SITL PR here](https://github.com/ArduPilot/ardupilot/pull/27536)) and for the moment at least I'm using the Blimp vehicle as the MAV_TYPE for the payload so this is another situation where it would be nice to have a circular vehicle appear.

Start by locating Mission Planner’s map vehicle-type handling and the existing multicopter icon mapping, then check the MAVLink definition linked in the issue for MAV_TYPE 7. Done means blimps or airships render with a distinct circular map icon instead of the multicopter icon, including the slung-payload use case described.

Written by the indexing model from the issue text.