Rename iced_sentinel to iced_beacon and refactor its API

beacon/src/stream.rs

@@ -0,0 +1,15 @@
+use futures::channel::mpsc;
+use futures::stream::{self, Stream, StreamExt};
+use futures::Future;
+
+pub fn channel<T, F>(f: impl Fn(mpsc::Sender<T>) -> F) -> impl Stream<Item = T>
+where
+    F: Future<Output = ()>,
+{
+    let (sender, receiver) = mpsc::channel(1);
+
+    stream::select(
+        receiver,
+        stream::once(f(sender)).filter_map(|_| async { None }),
+    )
+}