Implement XLIFF validation in a step

[Former user]

Original issue 199 created by @ysavourel on 2011-12-05T12:54:29.000Z:

Ee-using XLIFFChecker's code to add support for validating XLIFF documents would be useful.

[Former user]

Comment 1. originally posted by mihn... on 2013-01-01T22:26:12.000Z:

We now have xliff validation using schema as part of the checker.
Is that good enough to close this one?

[Former user]

Comment 2. originally posted by @ysavourel on 2013-01-02T03:33:26.000Z:

yes.