Fix toString and destroy on QObject JS bindings

The auto-tests requires these member functions to not rely on the
thisObject at all.

Also fixed lookup of QObject properties through the context wrapper
to go through get() as the new interface (to catch destroy/toString).

Change-Id: Iacbe8b5ce1895195a84a1224faba3f3bd34cd03b
Reviewed-by: Lars Knoll <lars.knoll@digia.com>