Translating region names (part 1)

In the current implementation, region names are not translatable (originally reported by kkaefer in http://groups.drupal.org/node/7843). To change this, we need three things:

1. Pass region names from system_region_list() to t() in block_admin_display_form() and template_preprocess_block_admin_display_form(). It seems that's the only two places where region names are displayed to the user.
2. Add the parsing of region[] tags in themes .info files to potx_find_info_file_strings() in potx.inc.
3. Manually add default region names to potx.

The following patch address the first concern.