Make it possible to call QQmlEngine::setObjectOwnership at any time

Make it possible to call QQmlEngine::setObjectOwnership at any time

QJSEngine::newQObject should call setObjectOwnership only if it wasn't
called before, to preserve any policy set by the user earlier. Otherwise
it's only safe to call after newQObject and then you'd have to call it
_every_ time the QObject is wrapped, at all call sites. Instead this
patch preserves any policy set.

Task-number: QTBUG-37160

Change-Id: I6178fa38cd7f4b09d818b6f18762c21588562422
Reviewed-by: Lars Knoll <lars.knoll@digia.com>