section.property cannot deal with nested properties

If a model contains roles with their own properties (such as objects) they cannot be used in section.property.

A roughly realistic example is modifying the qtpim contacts example (qtpim/examples/qmlcontactslistview) with the attached patch. This makes it try to add section headers by first name (contact.name.firstName), a data member used successfully in the delegate but not in the section headers.