Fix QObject ownership

Implement the JS vs. C++ ownership policy. QV4::WeakValue is a weak
reference that's now used instead of PersistentValue in QQmlData.
Whether or not to delete the QObject when the JS object is garbage
collected is decided in the ~QObjectWrapper destructor (conveniently).

Fixes four ownership tests.

Change-Id: Iedeb498f510295b5e656d0bb3b324084efa98f0f
Reviewed-by: Lars Knoll <lars.knoll@digia.com>