Problem to solve

Currently there's no way to display some custom content for a tree with no items ("empty" state) and when nothing's been found given some filter/search value.

Proposed solution

The proposed solution is to introduce 2 new slots: "empty" and "not-found", where they will, respectively, display their contents when the tree is empty and when searching in or filtering the tree items resulted in no items matching the given search/filter criterea.