Move the Swedish l10n

The locale for the Swedish version of Firefox is `sv-SE`, as can be seen here: http://mxr.mozilla.org/l10n-mozilla-release/source/sv-SE/.
In PDF.js the Swedish l10n is denoted `sv`, which means that when using the addon in a Swedish version of Firefox not all strings will be translated. The reason for this is that PDF.js will attempt to load translations for the locale `sv-SE`, which doesn't exist.

This PR simply renames the Swedish l10n directory to fix these issues.