Make it easier to read and write model data outside of a delegate

When in a delegate, it's easy to read (and have bindings to) and write model data:

Slider {
    value: model.someValue
    onMoved: model.someValue = value
}

If you want to have an editor that is equivalent to a delegate but only shows the current item, it's less convenient:

Slider {
    value: view.currentIndex !== -1 ? view.model.data(view.model.index(view.currentIndex, 0), Model.SomeValueRole) : 0
    onMoved: view.model.setData(view.model.index(view.currentIndex, 0), value, Model.SomeValueRole)
}

This assumes that you have an enum for the roles and that that enum is exposed to QML via Q_ENUM. It also doesn't react to changes to the current model data.

We should make this more convenient somehow.