[TASK] Change XLIFF to version 1.2

In order to be able to use Qt Linguist to easily translate
XLIFF files, one has to use version 1.2 and reference the
corresponding XML namespace.

Qt Linguist may be downloaded from:
http://qt-apps.org/content/show.php/Qt+Linguist+Download?content=89360

Change-Id: I82964087f072cb0eca2f7b783cfef9d746902bfb
Reviewed-on: https://review.typo3.org/26973
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers