Added implementation for NSUbiquitousKeyValueStore

[arthef]

Using NSUbiquitousKeyValueStore settings can be automatically shared between devices. The implantation follows convention of other implementations and is mainly based on the NSUserDefaultsSettings code.

Except standard initialization, no other special code is needed. However, to make key-value storage work, the app needs to be correctly configured in Xcode:

In the app project, TARGETS, Signing & Capabilities a new capability needs to be added (+ Capability):

1. iCloud, Service: Key-value storage

Also, it is highly recommended to add and implement external change listener(s) to make the app aware of the settings change on other device.

[arthef]

Hopefully with my recent commits all concerns are resolved. Please take a look and let me know if there is anything else to be done to accept the PR.

[russhwolf]

I'd like to get tests running against this as well. I can take a pass at it at some point but probably not soon.

[coreypett-cube]

Any ETA on this? Soon to fork and apply @arthef's work.

[russhwolf]

No ETA, sorry. I don't want to merge this without having tests running against it. Open to contributions there.

If you want it sooner you should be able to just copy NSUbiquitousKeyValueStoreSettings.kt to your project without needing to fork anything.