The "<all>" option

While working on http://drupal.org/node/216738, I figured I was doing it the wrong way: Views adds a special option with the value "**ALL**", so that's all I had to select. What I was writing, is a special "all" value that only appears in the hierarchical select and that, when clicked, selects all items in the original select.

I decided it was a worthy addition anyway. Posting this issue to clarify the reason for adding it.