added a file save indicator

fixes https://github.com/dart-lang/spark/issues/443

This:
- adds a save indicator to Spark, which toggles between 'Saving…' and 'All changes saved'. This is close modeled after Google Drive's UX.
- adds an event bus class to decouple the code which fires save events and the code which displays the save indicator UI

@dinhviethoa @keertip