Issues after webkit port:
 - PHOTO_MODE doesn't work for very tall pages (like pgo, see comment before
   gtk_widget_set_size_request()). We can possibly use a GtkScrolledWindow to
   make this work.

Webkit issues:
 - Using the custom CSS for iframes doesn't actually work perfectly: it seems
   to append lots of empty space at the end.

Improvements:
 - Improve thumbnail creation time
 - Thumbnailing service? I.e. background process (which exits after some time
   without requests), which the frontend tells which files/pages to thumbnail.
 - Fix the output on framed pages
 - Add memory size and CPU time limit (look at totem-video-thumbnailer/indexer)