TreePathSelected documentation and argument names

As I'm developing a project which uses Tree, I'm going to add some docs as I come across problems of understanding its workings. Here are some bits concerning tree selections.

I have renamed `newPaths` and `oldPaths` in `TreePathSelected` because they are confusing—they do not represented the old and new selections, but rather contain only those elements added and removed from the previous selection.

I'm also wondering what the purpose of the extra `TreePathSelected` extractor is. Having an element of type `Any` doesn't look useful to me. `Option[A]` would be useful, but probably there is no way to get that with the untyped reactors of scala-swing?