https://github.com/l0k1-smirenski/mdpreview-gtk3
The job was not nearly as hard as I expected. In fact literally, I had stopped work on it on the last, and only bug that stopped it running in the new environment. However, the interface is wrong.
Fortunately, you can even see what I mean, in the previewed markdown document, is a snapshot of what the interface should look like. Well, other than my gtk2 dark interface theme.
Furthermore, the process refuses to stop properly. So there is a few small kinks yet to clean up. I also need to add, and set as default, the steemit.com stylesheet in its stylesheet collection. I will have to dig through it quite a bit, rewrite the parts that generate the interface, switch them over to using HeaderBars instead of Toolbars, and discover why it's not killing the Gtk event loop process.
It shouldn't take too long to fix, the errors will mostly be in the gtk library calls, and I will have to rewrite the interface to use a toolbar that replaces the window decorations at the top. Not a big job. But once it's done, then I can move on to steemeditor! w00!
Edit:
Having looked closely at the coding style I am not in fact going to just try and fix up the problems it has, these problems are structural and need a total restructuring. By the time I am finished so little will be original code from the former project, and I'll be daring and take that GPLv3 licence off it and it'll be solely under the unlicence, 100% PD.