Use wp_dropdown_categories() instead of deprecated wp_dropdown_cats() in the WP backend

[markjaquith]

We need to replace all instances of wp_dropdown_cats() with wp_dropdown_categories()

Erm. we don't use dropdown_cats at all in the backend as far as I can see.

Or anywhere else for that matter.

wp_dropdown_cats()

Oops, yeah, meant wp_dropdown_cats()... too many similar functions :-\

Replace wp_dropdown_cats with wp_dropdown_categories

Ok now I see what you meant.

Only occurance I can see is in edit-category-form.php.

Fixed in attached patch which also fixes category walker buglet - padding was not included.

(In [4156]) Use wp_dropdown_categories(). Fix padding. Don't allow a cat to be it's own parent. Props westi. fixes #3088