Multiple fixed wing aircraft identification when using multipoint radios

Unlike with copters, when flying multiple fixed wing aircraft with multipoint radios the red airplane icons can not be distinguished apart. Ideally being able to select a different Icon for each sysid_thismav would be better than just displaying the sysid_thismav over the icon.

The issue names no files, tests, or entry points. Start by locating the fixed-wing aircraft icon rendering and the handling of sysid_thismav for multipoint radios; done means multiple fixed-wing aircraft can be visually distinguished without relying only on the displayed system ID.

Written by the indexing model from the issue text.