pimp the thumbnailer

* duplicate into two plugins:
  calligraimagethumbnail for preview without blending the icon (for all image formats),
  calligrathumbnail for preview with blending the icon (for all other)
* wait for result of thumbnail rendering if needed
* install separate thumbnail descriptions per import filter and only if
  the import filter is installed
* put a white background behind extracted mimetypes

REVIEW: 105373
BUG: 290875
FIXED-IN: 2.6