StaX filters now integrated in standard OmegaT

Jean-Christophe Hellary asked me last year whenever it would be possible to integrate our filters for XLIFF and OpenXML in standard OmegaT. This required some work but it is finally done.

For the moment my choice was to work with branch 2.0 of StaX filters, since branch 2.1 contains experimental features. It should remain useful to do tests with 2.1 (but then with OmegaT 5.7 or previous version!), if you consider the new features are interesting I will study their integration in OmegaT 6.1