Add a views filter/argument for Term Depth

I would like a view that can show all my taxonomy terms that are either the top level, or the 2nd level, but not below that.

In spite of there being a very complicated filter for "Content has taxonomy term with depth X" there is not a more simple one "This term has depth X".

I spent a while today trying to create the simple one based on the more complex one, but I'm not sure I can do it. Posting what I have here as a start for feedback, and so I can return to it later with fresh eyes.

Related drupal issues: * https://www.drupal.org/node/903438 * https://www.drupal.org/node/1073170 * https://www.drupal.org/node/1216006