Refactor l10n. Fix #363

Helpful links:

- <https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Reference/Interface/nsIStringBundleService>
- <https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Reference/Interface/nsIStringBundle>
- <https://github.com/adblockplus/adblockplus/blob/03b965c743f7688622b75b0e7525a17913d7e0ea/chrome/content/ui/i18n.js>
Merge pull request #365 from lydell/refactor-l10n

Refactor l10n. Fix #363