- use a c++ gui toolkit as you mentioned ( I would pick up Qt )
- expose your c++ engine to a high level language. This can easily be done by using swig (java, python, c#, ruby) or by creating a c++/cli layer (c# only)
- use inter-process communication between your editor and the game ( tcp sockets )
Generally speaking, the only things that have to be done to render a scene in an editor is to pass the editor window/panel handle to your c++ engine and call your render function periodically or when the application is idle.