Add LibreOffice checker

LibreOffice is diverging from OpenOffice so needs its own checker, right
now the checker is identical to the OpenOffice one, but changes will
follow.
Add new xml test and test for this test

Test translated strings belong to LibreOffice's
(and OpenOffice) help and readme, wheather xml/html
tags has their own open/close tag. This is the most
common build breaker problem with them.
Plus add test_validxml for testing
Adapt the previous commit to LibreOffice checker

And fix the coding style a bit.