make cart extract method not dependant on immediate parent.
make cart extract method not dependant on immediate parent.

TheVirtualLtd/bda.plone.cart,TheVirtualLtd/bda.plone.cart,andreesg/bda.plone.cart,andreesg/bda.plone.cart,andreesg/bda.plone.cart,andreesg/bda.plone.cart,TheVirtualLtd/bda.plone.cart,TheVirtualLtd/bda.plone.cart