Use --fg-aircraft paths for AI model

Use --fg-aircraft paths for AI model

Firstly search model in $fg-root/AI/ path,
if not found, search model in --fg-aircraft paths,
if not found, search model in $fg-root/Aircraft path

This commit should solve the complaint "I have an aircraft in a separate Aircraft/ folder
but during MP session other user are shown with the aircraft from the $fg-root/Aircraft path
instead of this behaviour I want to see the aircraft from my separate Aircraft/ folder"