Batched add in DirectPhotoSourceCollection causes a long delay until switching to next photo is possible
When using Shotwell's direct-view to look at a photo in a directory which contains lots of photos and/or some raw-photos, it takes quite a long time until it is possible to click on the next-photo-button.
I could fix that by removing the array-list prepared_photos in DirectPhotoSourceCollection and made on_file_discovered call add of the base-class DataCollection for each file the monitor reports (see attached diff for details).
My girlfriend asked my to solve that annoying behaviour she encountered very often while editing photos. I could find and fix the problem very quickly for her with this patch. But I must admit that I did no in-deep-testing for regressions. Neither have I tried to find out why that batching was in place. (do the signal-handlers of notify_items_added or notify_contents_altered take that much time?) So maybe you have another idea to solve that problem?