Accessing component data

• TModelSelection provides the interface for specifying a model. Subclasses of model selection provide for accessing and modifying specific data in that model. Each model should provide a selection subclass that works with the model's underlying data.
• TPresenterStateSelection provides the API for specifying data in a presenter state.
• TDocumentComponentSelection is a monomorphic class that is simply a model selection and a presenter state selection. A selection on a component can include entire components embedded in the selected component, but it is not possible to create a selection that partially selects multiple models. Specifications identify the data in an address-independent manner, so that selections can be applied across sessions.