See! This is what I've meant! :)
Nifty gives you an easy to use framework that you can extend with your own ideas. Just like Nifty user zozo64 did. Nifty is missing drag and drop support? No problem! He created his own! :D
To be honest, he found some bugs too :) but all of that improved Niftys core systems. So that's a good thing. We'll probably bring his work as a standard drag and drop control into the standard controls project soon.
For the moment you can take a look at the code. It's part of the nifty examples project: