The addons.Addon instance (ghost) is much more interesting than its base
path. The base path is immediately accessible from the addons.Addon
instance using its 'basePath' attribute. The addons.Addon instance
allows main() to easily access essentially all of the add-on metadata,
including the add-on identifier, which makes it possible to write
main.nas without hardcoding the add-on id at all.
This is an incompatible change of course, so better do it now than
later.
Also:
- use the namespace __addon[ADDON_ID]__ when loading an add-on's
main.nas file (previously, the namespace used was __addon[i]__ where
i is 0 for the first registered add-on, 1 for the second one, etc.);
- use logprint() instead of printlog(), because the former writes a
more helpful source file and line number for the log call in
fgfs.log (i.e., not always src/Scripting/NasalSys.cxx...);
- remove the listener once it has been fired;
- add documentation in Docs/README.add-ons.
