fixes for a couple of issues, some improved textures/cloud model changes,
smooth instead of hard visibility transitions, updated documentation.
Also removed 'Test' entries from the menu.
(ThorstenB: Also cleaned-up Docs/ folder, moving local weather
documentation images to sub folder).
I came upon the TODO asking for illustrations in the 3D Aircraft Models
HOWTO and thought "that's something I can do now!" :)
Merged into model howto by Ron Jensen.