Multiple Device Sync Support

This has previously been requested but I think has been misunderstood on what's being asked. While your app does automatically back up it's progress from a database perspective, it does not currently share this database across separate devices. This is something that needs to be explicitly setup. 

As an example, I currently have your app running on both an iPhone and an iPad. Any progress made on the iPhone is not actively synchronized to the app instance on the iPad. The cloud sync that you currently have is simply maintaining a backup of the app in the case of needing to restore a device. 

Here are links to the developer documentation and sample code for using native CloudKit, which would enable the functionality I am referring to:

https://developer.apple.com/icloud/cloudkit/
https://developer.apple.com/documentation/cloudkit/
https://developer.apple.com/documentation/cloudkit/enabling_cloudkit_in_your_app
https://github.com/apple?q=cloudkit-sample&type=repository

I would greatly appreciate a way to sync progress between multiple devices and it would bring another layer of functionality to an already amazing app. I know myself and many others would find it incredibly helpful since I'm often switching between my phone and an iPad when I study. As it stands currently, there is no way to do this concurrently and have progress match between devices, which becomes quite frustrating.

If I can help with testing or need to go into any further detail, I'd be happy to! Thanks so much for creating Ringotan, it's helped me so much in my learning progress.

- Andy