Allow the global JS UTC offset to be invalidated

Allow the global JS UTC offset to be invalidated

If the system timezone is changed, it is necessary to inform
V8 so that the global offset-from-UTC value can be recalculated.
This changes adds the 'timeZoneUpdated' function to the JS
'Date' class exported by QML, which allows QML applications to
inform V8 when the timezone has been updated.

Change-Id: Ic5898ca7bc640002a4a6fd5a52b8623d87ee8085
Reviewed-by: Martin Jones <martin.jones@nokia.com>