Improve QJSValueIterator implementation.

Improve QJSValueIterator implementation.

The old implementation was a hack, it had some memory leak (in case of
deleted engine) and performance problems (for example all names were
copied to separate QList instance instead of reusing v8::Array).

Change-Id: Ic70ad511127a8c05df3c627e4496083004c6452a
Reviewed-on: http://codereview.qt.nokia.com/2512
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Kent Hansen <kent.hansen@nokia.com>