Equip QDateTime to distinguish the two passes through an "overlap" hour during a time-zone fall-back, such as the end of DST

While QDateTime can map a UTC time to a local time, or time in some specified zone, in the overlap period of a zone transition, and will know which repeat of the time it represents (to the degree that it has the right offset from UTC, and will give the right time-zone if toString() is called with a suitable format) , we provide no way to indicate which repeat to use when constructing a time in this interval. The existing implementation will pick one of them; the user can check its isDaylightTime() and addSecs(60 * 60) (or minus that) to fix it up, but there should be a way to tell the constructor which repeat to use, when the specified time happens twice.

Python's datetime objects handle this using a "fold" parameter, set to 0 to get the first pass and 1 to get the second; this also affects handling of times in the gap when a time-zone transition skips an hour (as at the start of DST); PEP495 specifies the full semantics. In general, fold=0 selects the UTC offset in effect before the transition while fold=1 selects the offset in effect after the transition. I propose to add a parameter with the same semantics.