Again, I made some very small progress with
the editor I'm writing. It now looks like this:

The most interesting new thing is the material editor tab to the left. It works just like the
property window but has some additional features, for example a control which shows the 4 textures of the currently selected material parameter entry (an
SMaterial structure for those familiar with
Irrlicht). This new control is quite special: Because I didn't want to need to copy texture data from the 3D device into main memory, I simply draw that control using the 3D device. And in addition, I'm mixing drawing here with the GDI, the black border and the separators are drawn using the Windows graphics device interface, later I'm also going to show some text on it. I think some other editors are doing this in a similar way, for example Maya or the Unreal Level editor.