Fix memory management issue with accessors

The earlier pattern of
    Property p;
    p.setSetter(new (mm) setterFunction);
    p.setGetter(new (mm) getterFunction);
carries the risk of the second allocation garbage collecting the first one.
Consequently we need to put these values onto the JS stack, using a simple
ScopedProperty wrapper.

Change-Id: Ib29ea3b1eab95595dd6dfbb86fea282d23e3d899
Reviewed-by: Lars Knoll <lars.knoll@digia.com>