I've been thinking of making a cross-platform valve hammer editor replacement in C++. I know Jackhammer exists, and I use it extensively, but it is not open source or free (and nobody uses the out of date free Jack, I hope.) Trenchbroom could be a viable option, but half-life support is still under development, and is quite different to hammer. (I'm aware there's a four-pane view, I personally use that.) There may be an other option out there that I am not aware of, but as far as I know, something should be made addressing this "problem".
Here's what I think:
Qt | Seems like a viable option, but I cannot find any tutorials on this framework that don't utilise Qt Creator or Qt Designer. (This may be from a lack of research though!)
Dear ImGui | I don't think this can be made to look like a normal application. (?)
Win32 API | Probably the fastest, but is not cross platform.
Gtk might work, but I don't know enough about this.