Adding new instruments is a pain

Adding new instruments is a pain because we're now forced to select the right parent from a gigantic dropdown list (and since the names are indented to form a tree, you can't just type the first letters either).

In mason, I could use my browser's search to find what I wanted and then click a button which said something like "add child attribute" which was a lot nicer and I'd really like it to work like this again. For that, http://musicbrainz.org/admin/linkattributetype/create needs to accept the parent attribute's ID as a parameter and an "add child" link needs to be added next to the edit/remove links.