This PR fixes the encoding of CDATA markers when saving a translation.
For simplicity, strings are now always included in CDATA. Htmlentities are preserved (not re-encoded) if they were already in CDATA markers, and decoded if they were not.
So the XML file is always valid, and the strings are saved like they were seen in the backend form.
This will fix #18 (closed) and also #19 (closed)