This new class mirrors the existing QLocaleXmlWriter and places the
two side-by-side in qlocalexml.py, rather than having the writing and
reading in separate places.
Made judicious use of transformed versions of mappings to save
repeated iteration of a mapping's entries to do lookups on fist
entries of pair-values; several (id, name, code) data-sets are
sometimes indexed by id, sometimes by name.
Reworked the default_map, that the complicated compareLocaleKeys()
used in sorting locale keys, to map IDs instead of names; the function
also needed the locale_map so that it could convert IDs to names,
which we can skip by going directly with IDs.
Task-number: QTBUG-81344
Change-Id: Iff6a97f7f0755b56dda70d8a6796ec074c558910
Reviewed-by: Cristian Maureira-Fredes <cristian.maureira-fredes@qt.io>
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