[macOS] Override insertNewLine method to listen to Enter key (#136)
[macOS] Override insertNewLine method to listen to Enter key (#136)

Currently, hitting Enter while editing a TextField doesn't trigger any actions since
insertNewLine isn't overriden. This change overrides this method to listen for this
action, and invokes TextInputAction.done on the platform channel. 
google/flutter-desktop-embedding,google/flutter-desktop-embedding,google/flutter-desktop-embedding,google/flutter-desktop-embedding,google/flutter-desktop-embedding