QDateTime - Extend fromMSecsSinceEpoch API

Add convenience methods for fromMSecsSinceEpoch() and
fromTime_t() to enable direct creation of other time specs
than LocalTime without the overhead of unncessary conversions.

For example instead of:
  QDateTime dt = fromMSecsSinceEpoch(12345).toUtc();
the following saves two conversions:
  QDateTime dt = fromMSecsSinceEpoch(12345, Qt:UTC);

This will improve the performance of the new QTimeZone class.

[ChangeLog][QtCore][QDateTime] Added convenience methods for
fromMSecsSinceEpoch() and fromTime_t() to take time spec to be used in
returned datetime.

Change-Id: I133635bfe3d35ee496a287257e13b2d600225a38
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>