The lists of month and day-of-week names usually currently do not
exceed 13 elements (incl. possible leap months), so don't use
QVector, use QVarLengthArray with Prealloc = 13.
This value may be increased when ISO week-as-month is merged, but
the container remains valid even with 52 "months" (though the code
that calculates its runtime size will naturally need to be adjusted).
Change-Id: I4ead897d933f89fc092850fcc22ca41da0a6ddc6
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>