Cleanup: Move function inside factory

It is not used in any other place and this simplifies code making it
more readable.
Cleanup: Just close and reopen files once
Cleanup: Remove unused code in ODF conversion

Appending the translated XLIFF file to the translated ODF file is not done,
probably because it complicates testing the roundtrip conversion.
WORKING ON IT: Add IDML converter
WORKING ON IT: Now it parses just the Content tags and skips CDATA
WORKING ON IT: Convert to PO instead of XLIFF works!!
WORKING ON IT:    Trying to get roundtrip conversion working