add QTextDocumentFragment and clipboard Markdown support

QQuickTextEdit::getFormattedText() should be able to have

#if QT_CONFIG(textmarkdownwriter)
    } else if (d->markdownText) {
        return cursor.selection().toMarkdown(); 
#endif

But that implies adding QTextDocumentFragment::toMarkdown().

QTextEdit has support for copying and pasting HTML, but not markdown yet; perhaps it could be useful to make it available as markdown flavor just in case other programs can also work with it

QQuickTextEdit::insert() should also be able to handle markdown.