Improve checkout admin

* improve checkout fields editor to show heading hierarchy better
* improve checkout settings layout and strings.
* improve checkout form fields column widths.
* improve positioning of edit options link for select, check and radio boxes.

New Screenshots
![Google ChromeScreenSnapz013](https://f.cloud.github.com/assets/1079437/399559/20f1f892-a876-11e2-9950-ed1730552635.png)
![Google ChromeScreenSnapz015](https://f.cloud.github.com/assets/1079437/399567/4f602050-a876-11e2-97f2-119f19ece1fe.png)