Changed pqiv's behaviour when a file is deleted

From the source code:

> It is a difficult decision what to do here. We could either unload the deleted
> image and jump to the next one, or ignore this event. I see use-cases for both.
> Ultimatively, it feels more consistent to unload it: The same reasons why a user
> wouldn't want pqiv to unload an image if it was deleted would also apply to
> reloading upon changes. The one exception I see to this rule is when the current
> image is the last in the directory: Unloading it would cause pqiv to leave.
> A typical use case for pqiv is a picture frame on an Raspberry Pi, where users
> periodically exchange images using scp. There might be a race condition if a user
> is not aware that he should first move the new images to a folder and then remove
> the old ones. Therefore, if there is only one image remaining, pqiv does nothing.
>
> If anyone ever complains about this it would be best to add an option to entirely
> disable this functionality.