Initial release.
- This brings Pillow support instead of relying on imagemagick or graphicsmagick, making thumbnail generation **much** faster than prosopopee thanks to parallelization via multiprocess,
- Video thumbnails are also done in parallel directly within one ffmpeg call,
- Progressbars are shown to display progress in gallery parsing, thumbnail creation and video and audio reencoding to let the user know it's working hard :),
- Cache is saved if an exception occurs or Ctrl+C is performed, saving the work that's been done so far,