- settimer() has been disabled in add-ons since FGData commit
e39ebd79e3505c4a; use maketimer() instead.
- Use addon.resourcePath() to specify the path to the directory
containing the sound files; this allows the add-on to still work on
'next' where fgcommand("play-audio-sample", ...) doesn't work with
absolute paths anymore (this is so since FG commit 8853fded2953959;
see the discussion starting at [1]). This will also work on 2020.3
when SimGear commit 8febf6b9f58e9 and FlightGear commit 60ff405efcc
are cherry-picked to the release/2020.3 branch.
[1] https://sourceforge.net/p/flightgear/mailman/message/37694083/
