The various information (width, height, image type and number of layers) are
those of the full image, not of the thumbnail. Make it clear in the docs of
GimpRunThumbnailFunc.

Additionally:

- file-xmc was returning the proper information but variables were wrongly
  named, which was confusing.
- Fix file-ico thumbnail proc which was returning the thumbnail width/height.
- In file-darktable, initialize width/height to 0 so that we just don't show any
  size when we don't get the information. It's better not to show anything than
  completely wrong information (the thumbnail target size).