Import comments from SDLXLIFF to translate5

So far sdlxliff files with comments can not be imported at all.

This is changed with

TRANSLATE-1731 Ignoring of SDL comments in SDLXLIFF files

In addition with this new issue here it should be possible, to not only ignore the comments, but import them as translate5 comments. If the comments are imported or ignored should be decided by a system configuration.

Ignoring means unfortunately removing them completely from the SDLXLIFF file. Since comments can also be attached to parts of the segment, this info is currently getting lost. Therefore the comments has to be removed completly when ignored.

There is one caveat: translate5 has to delete the sdl-comment syntax from the segment in order to import it. In addition translate5 only supports comments on the target column on segment level - it is not possible to comment only a part of a segment in translate5 (exception: with MQM this is possible, but this is another technology). Therefore if a comment in sdlxliff refers to only a part of a segment, after reexport from translate5 the same comment will still be there, but will be attached to the complete segment and not just a part.

Since source content is not saved back into the SDLXLIFF file, the comment structure in the source column remains as it was before the import.