Provide better support for differentiating Repeater3D delegates

When instantiating with Repeater3D delegates, typically you want to make each one somehow different. Typically, you either bind delegate properties to some function providing the value either based on randomness or the index of the delegate, or directly bind delegate properties to derive their values based on the index of the delegate or data properties of an attached model. The first one is not possible in .ui.qml files, as you can't have functions (and they don't seem to work in editors in plain .qml's either), and the second one has no UI support, so you just have to know the index or the data property exists and can be used in bindings if you manually type it in.

I'm not sure what can be realistically done with either - I don't know what would be involved in providing support for functions, both defining them via UI and just having them work in the first place. And index/model data property UI support would require accurately detecting we are dealing with a delegate's property when defining the binding, and then having the index/model data property selectable in UI somewhere.