Add parameters to QFileInfo's time-stamp methods to configure time-spec and details

At present, QFileInfo's time-stamp functions – birthTime(), metadataChangeTime(), lastModified(), lastRead() and the fileTime() that unifies them – all coerce the result to local time. The underlying file engine may have produced them in local time or UTC time. The client code may prefer them in UTC time; for example, QDirIterator, when sorting by lastModified(), converts to UTC. Conversions between UTC and local time are not free, so the potential conversion from UTC via local time and back is wasteful. It would, in any case, be a kindness to users of these APIs to let them select the way the time is described in the call to them, quite apart from potentially saving some needless conversions.