0f08a911aa
Adds `addApplicationDelegate:` to the macOS plugin registrar, following the corresponding iOS method, and wires it up to the existing app delegation forwarding that was recently added for use at the application level. (The actual delegate is non-trivially different between iOS and macOS, but that's not resolveable without a complex migration on the iOS side, so the APIs currently diverge after the level of the `addApplicationDelegate:` method itself.) This doesn't add any new methods to the delegation; those will be added in a follow-up PR. Also fixes a retain cycle in the termination handler that prevented the new test from working. Most of https://github.com/flutter/flutter/issues/41471 [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style