add shortcut for "next photo" and "previous photo"

[Cobra]

It would be nice to have some shortcut to switch photos in agpifoj. This would speed up the work flow a bit and add a lot of comfort (at least for me).

I suggest using 'n' (at the moment this does the same as 'a' but isn't documented in the tooltip) or 'shift-n' for the next and 'p' (or 'shift-p') for the previous one.