Add transitions + new photo queue

Ok, these commits add:

* A "debug" mode (hit D on your keyboard)
* Simple crossfade
* "Flash" effect when displaying new images for the first time
* New images are also added to a separate queue that is favored when picking the next image to show. So when a new image appears, it is shown immediately. If more new images appear while it is visible, they are shown in succession. The slideshow then picks up where it left off before the first new image appeared.
* New images are kept on screen for slightly longer (if there's a big backlog of new images, the delay is reset to normal.