Tree widget: Add selectionPredicate to models tree building blocks

[grigasp]

Closes #1099.

The predicate receives a node and its type to help consumers decide whether selection should be allowed or not. Example:

function MyModelsTree(props) {
  return (
    <ModelsTreeComponent
      {...props}
      selectionPredicate={myNodesSelectionPredicate}
    />
  )
}

const myNodesSelectionPredicate: ComponentProps<typeof ModelsTreeComponent>["selectionPredicate"] = ({ node, type }) => {
  if (type === "element") {
    return true;
  }
  return node.label.includes("whatever");
};

[MartynasStrazdas]

Perhaps we could visually show which nodes are not selectable?

[grigasp]

Perhaps we could visually show which nodes are not selectable?

I guess that would be nice for users to know that before clicking on the node, but I think the visual should be very subtle to avoid extra noise. Got anything in mind?

Note: the primary use case for this feature is to allow consumers temporarily disable selection of specific nodes (e.g. subjects, models, categories) until other components are able to handle that stuff being selected. As such, I don't think we should invest much time into improving UX for it - the change should be simple to implement.