This adds AppKitDemo to the Examples project

Current setup:

• Uses a simplified schema from Reminders (but it could be the full schema)
• Uses similar patterns as other demos for Observable models and views
• NSSplitViewController at the app root for sidebar/detail
• NSOutlineView driven by observation of FetchAll for the sidebar
• Detail shows the selected item. Right now I used SwiftUI just to keep it simple.

Possible features to add:

• Sort and/or filter completed reminders in sidebar
• Mark reminder completed in detail
• Edit reminders list title in detail (and/or sidebar)
• Edit reminder title in detail (and/or sidebar)
• Add new reminder to list
• Add new reminders list
• Start with empty database, allow to seed like Reminders

From there we'd get into replicating more advanced features of Reminders, and I'm

Open Questions

• Is the scope of the demo good? I didn't want to get into replicating Reminders completely, but rather just to show a few ways to interact with AppKit. We could also use a totally different domain model.
• Should it be pure AppKit or is a mix of SwiftUI ok or even desirable?