For the new Settings dialog box, I needed an appropriate icon, and I found a generic one in the openclipart.org collection. Trying to add it into the nuvola.dll library which has all my icons, I discovered that the script that renders svgs into 12 different bitmaps fails on GIMP 2.4. After some research, I discovered why: variables must now be declared before being set. After adding declarations, it worked on Windows and on Linux. So I was able to add the new toolbar icon to the dll, and rebuild the dll.