Markdown backticks stopped working with recent Qt

[KitsuneRal]

When using pretty much any Qt that Quaternion still compiles with (Qt 5.15 and later, incl. Qt 6), QTextDocument::toHtml() no more asks for monospace font family, using the default monospace font family of the current system. This broke HtmlFilter::filterTag() that was detecting monospace among font families to put the respective text inside a <code> element. Some other way of checking for a monospace font family should be devised.