StaX filters : new release and plan to integrate to OmegaT core

Download:
    Test release (2.0.7): Source / Binary
    Development release (2.1.4): Source / Binary

OmegaT team is interested to integrate StaX filters in the core. A pull request is ready if you want to check for it. At same time we continue our developments here and found new bugs.

Small bugs have been revealed by automatic SpotBugs script integrated in OmegaT 4 core.

At same time correction made in previous release related to missing target was not totally correct: if there was no target in the source file but there is a translation in OmegaT, then this translation should come to the target file!