Add Blimp icon support

[rmackay9]

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.

In addition I'm working on adding support for reducing the oscillation of a slung payload (see SITL PR here) 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.