The Visual Editor of The Black Toolkit was made to work with many programming languages, and be extended for other needs of RAD Visual programming...
The good part is: ...just with XML.
The Componentset API is here: http://sourceforge.net/projects/theblacktoolkit/files/188.8.131.52/Extending_XML_Visual_Editor.pdf/download
Acctually the Black Toolkit supports (natively) is Web documents (HTML, PHP, JSP, ASP), Java Swing with NullLayout. But you can write the componentset for your favorite language, without recompiling the IDE.
I have too much work in developing the toolkit plus writing the componentsets. Because of this I had to drop the Win32 Form, to work on it some months later. Someone wants to help me?