QDateTime - Improve and expose Qt::OffsetFromUtc

The Qt::OffsetFromUtc TimeSpec was made public in Qt 4.4 but the access
methods were never made public in the apidox effectively meaning the
feature was never used.  The implementation was also incomplete and
inconsistent.

This change cleans up the implementation and exports new public API for
using the TimeSpec using new method names consistent with the new
QTimeZone support.

This change increases the QDataStream Version number for Qt 5.2 to 15.

The behavior of one constructor has changed slightly to be consistent
with the rest of the feature, but this behavior was never documented.

[ChangeLog][QtCore][QDateTime] Fully implement support for Qt::TimeSpec
of Qt::OffsetFromUTC, added new methods for offsetFromUTC(),
toTimeSpec(), and toOffsetFromUTC().

Task-number: QTBUG-26161
Task-number: QTBUG-29666

Change-Id: If3cc7fc9778ca2b831644408ae749448d5975a3a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>