Support Countries module (D7)

As already described in #1102662: Option to hide the country code when there is only one selected , the countries list is not very practical.
In D7 the Countries module contains Widgets to select only 'enabled' countries. It also re-uses the core-Drupal countries list.
A dependency upon http://drupal.org/project/countries would be welcome.
The countries table could also be a candidate to store the prefix and/or the validation expression.