Support for CSS classes

[shirakaba]

See:

• https://doc.qt.io/qt-5/stylesheet-reference.html
• https://doc.qt.io/qt-5/stylesheet-syntax.html
• https://forum.qt.io/topic/17237/using-css-class/6

The forum post suggested this approach:

parent.nativeView.setProperty("class", "my_supa_button");

*[class=my_supa_button] {
    font: 50px;
    color: black;
}