There is Sciter. I've been using it for a few months now and I ABSOLUTELY LOVE IT! I used to use pure win32 GUI programming, and I know my way around it very well, but it's unquestionably unwieldy. Then I stumbled upon Sciter (can't really remember how), and I've in love with it ever since. I will never get back to pure win32 programming after this.
I'm not affiliated with it in any way, but I honestly think it's an awesome library that deserves to be much better known. It has just got a new website a few days ago, so you will find the community forums rather empty unfortunately, and the developer is still working on improving the documentation. But the library is quite mature and has been around for a good while now.
Good luck. I think working on an engine along with its editor makes much more sense than creating an editor as an afterthought.
Sciter is amazing, I have been using it for 1 hour now and it is so simple to setup and use. This is by far the best UI framework I have ever tried.
Thank you :)