Optionally allow auto-advancing offline

[Lurux]

Currently, Muzei is only able to auto-advance when online, but it is also capable of advancing offline using the "skip" button. I think this option could be added in the menu:

When offline, just like when using the "skip" button, Muzei would show preloaded images if any are available, then fall back to random selection.

Ideally the "last downloaded" timestamp should not be updated, so Muzei can fetch fresh images as soon as the device comes online, even if it shouldn't change the image yet.

Along with this, I think it would be good to preload a few more images than currently, especially for the people who using that option: it would allow to get fresh images even if the device is offline for a small amount of time.