Investigate moving away from DO (Distributed Object) as IPC mechanism

MacVim currently uses Distributed Object as a way to communicate between individual Vim processes and MacVim but it's long been deprecated by Apple. See https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/DistrObjects/DistrObjects.html. As such, all usages of NSConnection are currently throwing deprecation warning.

Instead, Apple's preferred APIs are the XPC system: https://developer.apple.com/documentation/xpc

We should investigate whether to move off DO to either Apple's XPC or another IPC mechanism (for example, Neovim has its own IPC mechanism via msgpack). Since the DO implementation is the underlying system that we talks between MacVim GUI and Vim process, and there are a few quirks and back-and-forth messaging, we should make sure it keeps working (some edge cases includes window resizing with/without guioption-k, fullscreen, input, etc), and latency won't be affected as a result.

Also, occasionally we see error messages with DO (#1005), and I have seen issues with running unit tests locally if I have certain settings that seem to stem from DO init race connection, so hopefully we can eliminate those as well.

まず、MacVim GUI と Vim プロセス間で既存の NSConnection と Distributed Object の使用箇所を特定します。Apple の XPC ドキュメントを読み、issue で言及されている代替 IPC アプローチと比較します。実行可能な置き換えを特定し、通信、ウィンドウのリサイズ、フルスクリーン、入力、レイテンシ、報告されている接続エラーおよびテストの競合状態を検証できれば完了です。