Create thumbnails for the image collection as they become visible;
  cache the thumbnail in the `struct file`
  (also store the thumbnail size used and whether embedded transformation was applied)

Save preferences to file

Do not all of populate_collection() when new files are added without removing old files

Automatically go to next picture

Animation support

Colour space support

Add support for bicubic interpolation

Add support for Lanczos resampling

Need another library to display more SVG metadata