Fix timezone information being lost when creating events
setDtStart() was doing nothing even though the timeSpec was changed.
QDateTime::operator== sees the same moment in time when comparing "12:00 Qt::TimeZone Europe/Paris" and "12:00 LocalTime", so it returns true.
However in kcalendarcore, very likely since commit 670ebe21, Qt::LocalTime means a floating event, which isn't the same thing as an event with a timezone.
The resulting ICAL serialization was missing timezone information completely.