QDateTime - Fix tests to correctly restore time zone

The unit tests were caching the original TZ value to restore later
after testing with different TZ values.  The problem is reading TZ will
return a null value if no override TZ value is set, and if you then set
the TZ to null the system assumes UTC and not the system time zone.
Instead we need to unset TZ if it was null to start with.

Change-Id: Ib0625b1712e565f9fdfa99e2ffe1e5d74f059354
Reviewed-by: Mitch Curtis <mitch.curtis@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Doc: Renamed list of example page to "Item Views Example".

Previous name caused a conflict with another page with the same name.
This page will replace the one in qtdoc.

Task-number: QTBUG-32580
Change-Id: Ifda8841feaab4def6c16972a027947f75ccb7739
Reviewed-by: Martin Smith <martin.smith@digia.com>