Earlier quoted context omitted.
For Python, wxPython is good. Dropbox desktop clients are written in it, IIRC, and so are many other apps. Take a look at the demos in the Docs and Demos (separate EXE) that you can download from the wxPython site. Pretty good and gives an idea of wxPython's capabilities. For C++, wxWidgets (which wxPython is a wrapper for) or Qt should be good. Andy Brice, mentioned as a desktop product ISV in this thread, uses C++…
Is wxWidgets still under active development? The last 'stable' release was in 2014.
Sorry, only saw your question now. Not sure. Need to check.
Update: both Wikipedia and the wxWidgets site show a release this Feb 2016.