Binding Groups and Index to model

[indolering]

A lot of data already has a group or unique id embedded in it, or can be created by concatenating multiple fields. This would make it trivial to group items based on attributes of the model. We could also build an alternate index which can be used to perform scroll events without having to reverse engineer the list's index.

[indolering]

It would also be nice if the physical elements used a Map so one can easily check if a given item is rendered.