I have found a bug that I was searching for since a long time. Now finally, the level-editor works - well, at least as long as you do not use OpenGL in the mean-time (this should be just a minor bug, I assume some confusion of the standard input by OpenGL or something).
Even though there are a lot of alternatives, I think I will stay with ltk and skippy for the level-editor, simply because the level-editor was meant to be a development tool for me rather than part of the software.
The background is still not working and currently I do not have the time to make it work, though it is probably easy.
Anyway, if you want to have fun with it, or get inspiration (or just look at CL-Code dealing with OpenGL), it can be found under https://github.com/dasuxullebt/uxul-world