QTextDocument::toHtml silently ignores block markers

See attached example.

If I create a document, add a `QTextBlockFormat::marker` to some list element and then save it to HTML, the marker is silently dropped. The produced HTML is exactly the same as if no marker is set.

It can be easily reproduced with Qt TextEdit example application: just add a "Task List (Checked)" element, save document and then load it. The task list element is dropped.