Allow user to supply their own tree2 id scheme using new TreeWalker interface

The current HtmlTreeRenderer forces an id naming scheme of "0", "0:0", "0:1", etc. This can be problematic with dynamic trees where the ids of two different nodes may end up when nodes are constantly being added and removed in between requests. The new TreeWalker interface will allow users to supply their own implementations so they can override the node id algorithm with a custom implementation.