Implements the common functionality of groups and items.
NavBarControl maintain collections of groups and items. Groups and items have some common features. For instance, each can display a text caption, large or small images, etc. These common features are implemented in the NavElement class.
The NavElement object supports the ICollectionItem interface since both of its descendants serve as items within collections. The NavBarGroup class represents an item of the NavGroupCollection collection. The NavBarItem class represents an item of the NavItemCollection collection.