Validate XLIFF using schema

[Former user]

Original [issue 263](https://code.google.com/p/okapi/issues/detail?id=263) created by @ysavourel on 2012-08-07T00:11:02.000Z:

Enhancement: validate XLIFF using schema (xliff-core-1.2-transitional.xsd) Currently QualityCheckSession does not complain about the structure, unless it is an invalid XML.

[Former user]

Comment [1.](https://code.google.com/p/okapi/issues/detail?id=263#c1) originally posted by @ysavourel on 2012-08-07T00:18:26.000Z:

See email thread: https://groups.google.com/forum/?fromgroups\#!topic/okapi-devel/hDqA25qFYsM

[Former user]

Comment [2.](https://code.google.com/p/okapi/issues/detail?id=263#c2) originally posted by @ysavourel on 2012-08-07T00:19:49.000Z: