Introduce a BASIC_CATEGORIES variable at the top of
python3-flightgear/flightgear/meta/i18n.py that lists all categories
handled by BasicL10NResourceManager. Since currently all of our
translation files can be handled this way (the XML master files all have
a flat structure), we only need to say BASIC_CATEGORIES = CATEGORIES.
With this change, adding a new basic category such as
'weather-scenarios' only requires one to add it to CATEGORIES.
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