This is a shiny new widget that can display a tree using Miller columns ^:)
In many cases, the columns view can be used as an alternative to tree view,
but it has its own set of limitations:
* It can only display one model column (so it cannot replace a table)
* It takes up a lot of horizontal space, so it's only suitable if the item text
is fairly short
* It can only display one subtree at a time
But as long as a usecase doesn't suffer from these limitations, a columns view
can be *much* more intuitive than a tree view.
