Cannot easily remove popular category from a post

[mdawaffe]

To Reproduce:

1. Edit a post that is categorized under some "Most Used" category. Let's assume that category is called "Bartleby".
2. Uncheck "Bartleby" from "All Categories".
3. Save and Continue Editing.
4. Notice that the post still has category Bartleby.

In the above example, Bartleby is still checked under "Most Used".

Attached syncs the two checklists:

1. Most Used checklist starts with nothing checked and is hidden from users without JS.
2. On load, the Most Used checklist is synced with the normal checklist.
3. Whenever a checkbox is checked, that checkbox is synced between the two checklists.
4. Most Used cats added by AJAX (can happen if adding a sub cat or typing the name of the cat in the AJAX input rather than looking for the checkbox) are told to sync as well.
5. Only the checkboxes from the "All Categories" list are sent when saving a post.

Attached introduces:

1. New CSS class: wp-no-js-hidden which is set to display: none;. On load, the class is removed from all elements via JS.

Also Fixes:

1. Add new category "submit" link now a button.
2. Parent Category Dropdown selectedIndex bug.

[matt]

(In [6597]) Fixes to category list saving. Hat tip: mdawaffe. Fixes #5631

[mdawaffe]

testing trac's inability to display diffs