[ChangeLog][QtCore][QTimeZone] QTimeZone is now always defined;
feature timezone now controls most of its prior API and some new API
is added, most of it always present, to enable QTimeZone to package a
Qt::TimeSpec and, for Qt::OffsetFromUTC, its offset. Prior to this
change, APIs using Qt::TimeSpec had to provide a separate function
taking a QTimeZone alongside a function taking a Qt::TimeSpec and
optional offset; it will now be possible to unify these into a single
function taking a QTimeZone. Adaptation of other Qt classes to do so
shall follow.
Task-number: QTBUG-108199
Change-Id: If5ec3cc63920af882ebb333bf69cde266b1f6ad7
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
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