QxtGui is part of Qxt [libqxt.org], a third-party library not created nor supported by Nokia.

Indeed, not supported by Nokia, but available as open source under a liberal licence. Nothing is stopping you from using it.

You mean in the designer part? AFAIK, there is no designer plugin for the GUI elements in Qxt. However, you can use the widget promotion feature for most widget, as most of them are just extensions to existing widgets.

Qxt itself has quite decent documentation on how to use it in your project. It installs itself in such a way, that you can directly use it from your .pro file.