David Faure authored Dec 12, 2021 and Allen Winter committed Mar 06, 2022

12:00 my time can be either
 * floating if timeSpec() == Qt::LocalTime
 * non-floating if timeSpec() == Qt::TimeZone (in which case the system timezone is set)

KTimeZoneComboBox didn't distinguish the two, returning systemTimeZone()
in both cases. This commit adds new API that works with QDateTimes so
that the combo can read/write both timeSpec() and timeZone().