Document new fields:
authors
maintainers
license/{designation,file,url}
url/{home-page,download,support,code-repository}
tags
Add precisions concerning how fields are parsed, a bit more structure,
etc.
Mailing-list discussion:
around https://sourceforge.net/p/flightgear/mailman/message/36155660/
The explicit typing shouldn't change anything for the parsing code in
flightgear/src/Add-ons/AddonManager.cxx, because for each field, this
code knows the correct type and explicitly asks for it (and only once),
but external code might include addon-metadata.xml in some prop tree,
and then the explicit typing is likely to give better results.
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.
