Fix connect/disconnect in QObject bindings

The hack of using ScriptOrigin to identify JS wrapper functions for Qt methods
is not possible for us to implement (or doesn't make sense). However the bindings
rely on this feature, so implement it using a semi-private property hack.

Change-Id: I6c7852f46916a9f549bf703d47546e7ddd75a1b1
Reviewed-by: Lars Knoll <lars.knoll@digia.com>