Add a representedObject property to SourceListItem.

Also add a convenience initialiser method which takes a represented object and item.
Implement -badgeValue and -setBadgeValue: on PXSourceListBadgeView.

These methods just call through to the underlying badge cell.
Implement an example model object, PhotoCollection.

This is used to encapsulate information about collections used in the view-based
example project. It is an example of using a representedObject property on a
Source List item model to avoid having to synchronise model changes with the
Source List item model.