The old Tree component in wicket-extensions is removed

"It was based on Swing APIs and many people complained about this. Use the new Tree component introduced in Wicket 6.0.0 instead."
Impact: small. Our JcrTree component is based on this old Tree component, which is used for all JCR node-based trees (content trees, console tree, etc.). We can either rebuild it entirely on top of the new Tree component, or use the wicketstuff.wicket1.5-tree project to pull in a copy of the old tree. The latter is way easier, so let's do that. We can always refactor the current tree later.