Icon sourceName does not work if application run via QGuiApplication

Suppose I wrote this piece of qml code:

ApplicationWindow {
    visible: true
    ToolButton {
        action: Action {
            id: boldAction
            checkable: true
            iconName: "format-text-bold"
            //iconSource: "textbold.png"
        }
    }
}

Now, if in main.cpp I will use

QGuiApplication a(argc, argv);

(which is from qml module of Qt), then there will not be icon in toolbutton.

But if I replace above QGuiApplication a(argc, argv); with

QApplication a(argc, argv);

(which is from old widgets module of Qt), all works perfectly.

So, my question is – is it a bug, or intended behaviour?
If it is intended then why QApplication is in widgets module of Qt? Must I still use widgets module (and classes from its module) for properly working qml?

Minimal example here