wp_category_checklist() does not treat popular categories correctly, does not work with ajax

[mdawaffe]

Attached

1. Makes wp_category_checklist() work with ajax add-cat call. Added args to only show part of the category tree.
2. Corrects get_terms call for popular cats. get_terms() isn't fully functional for fields=ids: manually disable some incompatible checks by setting hierarchical=0
3. Tweaks JS while I'm at it.

[ryan]

(In [7777]) wp_category_checklist() fixes from mdawaffe. fixes #6810 for trunk

[ryan]

(In [7778]) wp_category_checklist() fixes from mdawaffe. fixes #6810 for 2.5