Agenda: fix timezone being lost when moving event by drag
The code was using QDateTime::toTimeSpec() in attempt to restore the timezone from the original dtStart/dtEnd. However, QDateTime::toTimeSpec() automatically converts Qt::TimeZone timespec to Qt::LocalTime, so the information about timezone was lost and the time was treated as local.
The solution is to restore the timezone manually when the original timespec was Qt::TimeZone.
CCBUG: 448703 FIXED-IN: 5.24.1