Given your requirements I would say that the best choice would be wxWidgets
From your feature list:
- Cross Platform Compatibility Yes
- Open Source Yes with a permissive licence
- Active with Documentation Very active and comprehensive documentation
- Clean & Easy to Learn the one thing that just about everybody seems to have problems getting their heads around is sizers
- Mature & Active - has been in existence since 1992 & active hundreds of developers and thousands of commits.
- Also provides platform independent abstractions for many none GUI elements such as date/time, threads, sockets, etc.
But I would suggest also installing:
- Python - cross platform programming language
- wxPython - python bindings for wxPython
- The wxPython Docs & Demos package
The first is very useful all by itself, the second allows you to prototype your GUI rapidly and the 3rd provides Open Source, Editable, examples of the majority of the wxWidgets controls.
The nice thing about prototyping your GUI with wxPython is that as it is an interpreted environment you can get very rapid results and can try things out interactively.