Replicated images cache (previews/thumbnails)

[kirillt]

The app is generating previews/thumbnails and storing them in cache folder.

Since the app is supposed to be used coupled with replication mechanism (like Syncthing) we can avoid re-generation of caches if we persist them. It might be stored in hidden folder. Only relative paths must be used in this case.

In case of previews/thumbnails, we have to ensure that they are generated universally for any device — it doesn't make sense to replicate cache if it works well only on certain device. This may change in future, since it is good to generate different previews for different screen sizes. When it'll change, we would like to have preference in settings menu (#77) for disabling replication of images cache to other devices.