New Categories interface on Write -> Post

[mdawaffe]

Features:

1. Lists all cats hierarchically.
2. Lists most popular cats flatly.
3. Ajax cat addition now respects hierarchy. Cats added as a child show up as a child (but not at the top of the list - we can maybe work on that if desired).

Implemented using:

1. wp-lists.
2. jQuery UI Tabs. This is a little big for just this one-off thing, but it's very nice and we can surely use it many places. It's reasonably small when minified.

Known Issues:

1. Adding a subcat to a subcat can sometimes unparent the later.
2. Load page, type in new cat name, hit enter: Post form is submitted.
3. Add a subcat, new option in dropdown is "parent child".
4. New cats don't go into the right place in the dropdown tree (probably won't fix this).

(In [6588]) New experimental category interface, fixes #5618. Hat tip: mdawaffe.

5618b.diff

1. Fixes all above known issues in FF.
2. Changes some submit detection in wp-lists. Needs cross browser testing.
3. Moves gray in-input "tip" ("New Category Name", "Add new tag") to new form-input-tip class. Want to call it something else? form-inline-label?

(In [6591]) Fixes for new categories interface from mdawaffe. fixes #5618