Support MemoQ comments for im- and export

MemoQ unfortunately does not stick to standard xliff 1.2 syntax for comments, but has its own format.

Inside a trans-unit you find this syntax:

<mq:comments>
<mq:comment id="4a3f1dad-d417-4266-b832-6d6721e2e9bf" creatoruser="user1" time="2020-01-27T06:21:07Z" deleted="false" category="0" appliesto="Target" origin="User">Dies ist ein Kommentar.</mq:comment>
<mq:comment id="f7ae96cf-1e2b-4587-a16b-17a913dce70b" creatoruser="Table RTF" time="2020-01-28T09:22:33Z" deleted="false" category="0" appliesto="Row" origin="SourceFile">Dies ist noch einer. </mq:comment>
<mq:comment id="74d5b955-c68b-4ad3-8678-4d0af352eacb" creatoruser="Table RTF" time="2020-01-28T09:22:33Z" deleted="false" category="0" appliesto="Row" origin="SourceFile">und nochmal einer.</mq:comment>
<mq:comment id="2cf2933e-f7d6-4c33-82fd-3cc5811a0227" creatoruser="JF" time="2020-03-10T11:06:08Z" deleted="true" category="0" appliesto="Target" origin="User">Hallo Marc,</mq:comment>
<mq:comment id="4129e22f-406d-4ffd-bf0a-c2dc7e93ad6d" creatoruser="JF" time="2020-03-10T11:06:20Z" deleted="false" category="0" appliesto="Target" origin="User">Hallo Marc!</mq:comment>
</mq:comments>