I could not disagree more.
The task of this is not to create a game UI (which Qt would be completely wrong for, indeed, but for other reasons).
The task is to create an editor, akin to Maya, Blender, Max, Unity...
This is a completely different thing.
And you can completely forget about doing any of that with any of the libs you listed if you do not want to spend weeks after weeks of implementing basic windowing, widget and misc functionality, which you'd all get for free.
And that doesn't even cover that editor applications are expected to follow a certain OS-like style in their layouts (note how Eugene's screens so far make it look somewhat similar to a dark Visual Studio).
Or they will simply end up looking amateurish (as the current Urho editor does). And will be very hard and unintuitive to use.
Good luck designing that with CEGUI
I tried working with most of the libs you mentioned. They are horrible for anything more than very basic layout needs. And most of them come with no kind of layout designer (or pretty bad ones).
IMO the only viable GUI library for games which will not cost your sanity is do-it-yourself with libcef3 (as that offers all that can be done in a browser) or the commercial alternative that already did the integration (forgot the name to be honest...).
At a monstre 15gb download on my mac, it's as big as the Microsoft virtual machine for development, fully packed up with windows os.
I cannot believe that we still get this in 2017. 15gb is nothing. Get over it.
And besides, the actual distribution size of a Qt app is way, way smaller than that - an application I am developing that is using a lot of Qt functionality comes at a size of 200mb, of which Qt is not even 100mb.