[macOS] keyevent not implemented

[maun]

The keyDown/keyUp events which should be sent over the "flutter/keyevent" channel are not implemented in the macOS embedder. Because of that RawKeyboardListener does not work.

I got a working version in this branch: maun@b6bdb28

What would be necessary for a PR?

[stuartmorgan-g]

Cool!

You'll need to sign the CLA as described at https://github.com/google/flutter-desktop-embedding/blob/master/CONTRIBUTING.md but otherwise just go ahead and send a PR and I can review it.

(A quick note from glancing at it: it looks like all the new code is using four space indentation; code should follow the Google style guide: http://google.github.io/styleguide/objcguide.html. I'd recommend running clang-format over all the files you've edited.)

[stuartmorgan-g]

I'm going to close this as fixed, and remaining work can be tracked in follow-ups if/when anyone needs more functionality (e.g., modifiers) before #47 is addressed.