accesskeys: add properties to bad accesskey detector

Extended the script to check for bad accesskeys in .properties files
six: this is now a hard dependency

Previously our dependencies where all optional, but now we require six
to actually run the toolset.

Add requirements parser from Pootle.
roundtrip: ignore the noise created by roundtrip scripts
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.