Datetime optimization (#439)

Datetime optimization (#439)

* Optimize performance of DateTime.

Prefer Qt::UTC TimeSpec for storing creation_time.
This avoids very significant performance hits from converting
to/from Qt::LocalTime, which involve DST calculations as well
as the usual offsets.
Test suite with this commit runs at 63% of wall-clock time of the
baseline (time ./testo)!

* remove obsolete fromTime_t and use Qt::UTC.

* Delete obsolete operators and methods of DateTime.

* document motivation for Qt::UTC as default timespec.

Modify Waypoint::SetCreationTime so it can be used with seconds, milliseconds,
or both operands.  This method is implemented without resetting the
Qt::TimeSpec.  It will be efficient if the existing TimeSpec is
Qt::UTC as set up by gpsbabel::DateTime default constructor.