Problem with async external events: ExtEventSink

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The example code /druid/examples/async_event.rs has this problem, if you are using tokio instead of threads. Try logging the color_clone in front of the `event_sink.add_idle_callback` call and in function supplied to the `event_sink.add_idle_callback` call.

Some events triggered via add_idle_callback - if they are early enough - are ignored. This happens probably because the launcher wasn't yet launched with `launcher.launch(Color::Black)`.

There are two solutions for this problem:

- Create an `launcher..spawn(event_handler_func: fn (event_sink: ExtEventSink)` method to add external event sinks. The Function is called, when the `launcher.launch` is called.
- Handle `event_sink.add_idle_callback` which were added before `launcher.launch`

My current *hacky* solution is to wait three seconds before calling the event handler:

```rust
let event_sink = launcher.get_external_handle();

task::spawn(async move {
time::sleep(Duration::from_secs(3)).await; // Hack
handle_events(event_sink).await;
});
```

I'm using tokio so task instead of thread is called here.

Start with druid/examples/async_event.rs and trace ExtEventSink.add_idle_callback around launcher.launch(Color::Black), reproducing the behavior with tokio rather than threads. Compare the logged color_clone values before launch and inside the callback; done means callbacks registered before launch are no longer ignored or the supported launch-time handling is clearly established.

Written by the indexing model from the issue text.