Teach QDateTime to parse common time-zone offset formats from string formats

In cases when date-time format string includes a `t` specifier and the date-time string has corresponding time zone specifier which is a UTC-offset zone identifier, parsing fails if such zone specifier is not prefixed with "UTC" keyword.

For example: 2001-09-15T09:33:01.001-03:00 should be a date-time with Qt::OffsetFromUTC as time-spec. Corresponding 2001-09-15T09:33:01.001UTC-03:00 is parsed correctly.