No consistent way to refer to model data within a delegate

Model data can be provided to QML views in many forms: a ListModel, a custom QAbstractListModel-derived class, QObjectList, a JS array, etc.

Currently, a delegate must know which of these types of models is going to being used, as there isn't a unified way of working with collections of data. For example, with ListModel the delegate would need to use model.someData, while for a JS array of objects it would need to use modelData.someData (QTBUG-49983).

This is unintuitive, and means that models aren't easily interchangeable: for example if one type of model is used for simulation/prototyping, and another for production code.