Add all macos files generated with flutter create .

According to this documentation:

    https://flutter.dev/desktop#add-desktop-support-to-an-existing-flutter-project

This commit adds all of the files which came from running `flutter
create .` while on the master branch of flutter-sdk.

After this commit (and while still on the flutter-sdk master branch) the
native mac os app can be run with `flutter run -d macOS`.