Handle name collisions between required properties, existing properties and model roles

In 5.15, required properties can be used to explicitly extract model values. However, this currently requires an exact match between required property name and model role.

This can be problematic if the application has a model whose role names cannot be changed, and which collide with existing properties of the QML delegate type (e.g. a  model with a "width" role).

While this was already before required properties, and can be worked around by using model["roleName"] to access the value, it is clearly sub-optimal and we should provide a better way.