lookandfeel: add preview generator script
Usage example: ./generate_previews.py -l /usr/share/wallpapers/Next/contents/images/1920x1080.png -d /usr/share/wallpapers/Next/contents/images_dark/1920x1080.png
Usage example: ./generate_previews.py -l /usr/share/wallpapers/Next/contents/images/1920x1080.png -d /usr/share/wallpapers/Next/contents/images_dark/1920x1080.png