Integrate macOS documents with NSDocument?

### What is the problem or limitation you are having?

I don't see the native NSDocument class being used in Toga Cocoa. This native class (https://developer.apple.com/documentation/appkit/nsdocument?language=objc) has iCloud support for reading and writing documents and also renaming etc. supports.

I understand that Toga does not differentiate a document app anymore however the following dialog is expected by mac users in apps dealing with documents, and we should try to implement that.

### Describe the solution you'd like

When an application is a document app, somehow cooperate with the native NSDocument to manage the documents.

### Describe alternatives you've considered

Status quo, however as far as I understand that will make Toga document apps not integrateable with iCloud.

### Additional context

I'm sorry for the vagueness but as this has to do with apps and windows and stuff I am not in a position to propose a concrete solution.

The issue names Toga Cocoa and Apple's NSDocument API but no repository files, tests, or entry points. Start by reading the Cocoa backend's application and window integration alongside the NSDocument documentation. Done should mean a concrete design and implementation path for document apps, including the native document dialog and iCloud-related document management.

Written by the indexing model from the issue text.