Mac client directory reorganization

[colincornaby]

I think it's time to re-organize the Mac client directory. It's a good time to do so as we actively talk about how to do more code sharing with the clients. There's also several other reasons to look at doing so now.

• With iOS (and visionOS), I'm looking at adding a SwiftUI front end. We'll keep the AppKit front end on the Mac for now. But in the future the SwiftUI client could become the client for all Apple platforms.
• Swift is coming into both the AppKit and SwiftUI clients. Swift adds a bridging header - which is a single header for including any C/C++/Obj-C headers into Swift. I'd like to share a bridging header between the AppKit and SwiftUI clients.

I'm thinking that the directory layout could look like the following:

plClient/
   Apple/
      AppKit/
         ...
      BridgingHeader.h
      Shared/
         ...
      SwiftUI/
         ...

There are several open PRs involving the Mac client right now. I'd probably want to get those resolved before looking at opening up a PR on these changes.