> By desktop environment I meant e.g. Gnome or KDE. Aren't they separate from the widget toolkits?

I was being charitable. The desktop environment doesn't belong in the graph you drew. The GUI widgets belong in the place where you wrote "desktop environment". The charitable assumption is that you reasonably conflated the GUI widget toolkits for desktop environments, since the very popular KDE and Gnome are strongly tied to Qt and gtk, respectively.