Ensure JS-owned QObjects are cleaned up on v8engine dtor

Ensure JS-owned QObjects are cleaned up on v8engine dtor

This commit ensures that the garbage collector is invoked during
engine destruction.  This commit also adds a unit test which
ensures that the JS GC destroys JS-owned C++ objects correctly
when the QDeclarativeEngine is destroyed.

Task-number: QTBUG-20377
Change-Id: I2de1f2dfd1e60cc2f76abb523b99bf169d2a5a13
Reviewed-on: http://codereview.qt-project.org/3285
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>