Image drag and drop is not persisted

When an image is inserted in a document and then repositioned with default drag&drop implemented in the browser, it is not persisted in its new position.

This is because, upon the drag&drop operation, the only element repositioned is the <img> element, but the syntax marker comments stay in the old place. Just as described in XWIKI-2918, the empty comments generate the image and the new img element is not transformed in a wiki image by the parser.

This has been tested on FF, but I don't find any reason why it wouldn't reproduce in other browsers too.