This is pretty nebulous. Please be specific in what ways Qt is better than gtkmm in this regard.
Again, this is very nebulous. Be specific.
> structure of GUI components
Yet again, this is not at all specific. What precisely is better about Qt's "structure" than gtkmm's? List examples where you can.
> the designer IDE (here I'm not 100% sure)
This is simply a laundry list of things that are "better" (and most of the items aren't concrete examples of anything; they're categories) and nothing more. Please be specific with your argument and back it up with examples.
I'm going to learn Qt for maemo, so I hope to be less ignorant of Qt in the near future, as time permits. I do have a degree of experience in gtk and gtkmm, so I want to know what you think is failing (and perhaps even fix it). I can't fix "documentation"; I can fix the fact that there aren't any tutorials on how to frob the critical blort property in gtk/gtkmm, though.
> A GUI is composed of elements that in some natural way inherit from each other. E.g. there are push buttons, radio buttons, which are all buttons. And buttons and many other elements are widgets. The main windows, dialogs and so on are frames or windows.
Honestly, right here you sound like you haven't even looked at gtk+/gtkmm documentation, let *know* anything at all about it.
I think you've confused me for you. I'm wondering why you think gtk+ is inferior to Qt in all situations (implicit in the statement "[gtkmm is]That's not as good as Qt." which is without qualifications. I don't mind Qt at all, and would like to improve gnome/gtk.