Suggestion: stop dealing with the `model`

[chrisfls]

I disagree with how this library deals with the user's model:

• itemsFromModel
• selectionFromModel

My suggestion is to change them to plain items : List item and selection : Selection item.

I can't see any advantages in keeping those *FromModel functions, and they also add a cumbersome model type var everywhere.

... Of course, we could just use always to skip the model requirement, and this is what I've done when integrating this library with paack-ui. But the current API doesn't seem either ideal or standard.