This eliminates a deadlock in BackgroundJob and moves the next/previous controls into SinglePhotoPage along with the hacky fix we have for when the user holds down the next/prev key.

This does uncover two more issues, which I've filed tickets for.
1. The hacky fix is entirely time-dependent. On a slow enough computer it won't work. Ticket #3892
2. Slideshow transitions shouldn't be occurring when I use next/prev. It should just jump to the next photo asap. Ticket #3891