Just whipped together a project with gtkmm/libglademm that is not only fully-featured but runs under 300 lines of code. 300 lines of code.
I had read all of the crap spewed about these libraries, but just really gave them a chance and they are clean. If you know C++ and you use the STL (you do, don't you?) you should be able to pick up the libraries in no time. Now I just wish that they shipped by default on more systems.