Manual album sorting

[stumpylog]

I'm considering using PiGallery to display a number of old scanned slides and photos. Given these are scans, the metadata has no real meaning to sort the images. Therefore, it would be very useful to allow creation of albums where the order of the images is manually defined (by drag and drop, for example). This would allow sorting images based on the rough date and trip/location/etc, while ignoring the EXIF metadata.