QTextDocument exports unescaped control characters to HTML

The following code leads to a control character being exported from a `QTextDocument` as-is, which doesn't seem to comply with what W3C says, see https://www.w3.org/International/questions/qa-controls.

#include <QTextDocument>

void main() {

    QGuiApplication app;

    QTextDocument doc(QStringLiteral("\x1b")); // ESC character

    auto html = doc.toHtml();

    Q_ASSERT(html.indexOf('\x1b') == -1); // Will fail

}

To be honest, I don't know what should be the right course of action here, since toHtml() returns HTML 4.0, which doesn't support control characters at all. Perhaps I would expect stripping them away or at least (even if that's still not quite correct) turn them to `` etc.