Add contextual information on translatable strings.

Source String Contextualization Group (SSCG) is a Fedora SIG group
initiated to provide meaningful descriptions of the source strings
for translators to ensure the correctness and quality of the translations.

https://fedoraproject.org/wiki/SSCG

BUG=rhbz#1069594

Review URL: https://codereview.appspot.com/69010043
Patch from Nilamdyuti Goswami <ngoswami@redhat.com>.