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().