Iced port of status area applet

This is based on the GTK version of the status area applet that was
previously in this repository.

This exposes app indicators found over dbus. As used in applications
like nm-applet and steam.

cosmic-applet-status-area/src/subscriptions/status_notifier_watcher/mod.rs

@@ -0,0 +1,58 @@
+// TODO: Both this and server proxy could emit same events, have way to generate stream from either?
+
+use cosmic::iced;
+use futures::StreamExt;
+
+use crate::subscriptions::status_notifier_item::StatusNotifierItem;
+
+mod client;
+mod server;
+
+#[derive(Clone, Debug)]
+pub enum Event {
+    Connected(zbus::Connection),
+    Registered(StatusNotifierItem),
+    Unregistered(String),
+    Error(String), // XXX
+}
+
+enum State {
+    NotConnected,
+    Connected(client::EventStream),
+    Failed,
+}
+
+pub fn subscription() -> iced::Subscription<Event> {
+    iced::subscription::unfold(
+        "status-notifier-watcher",
+        State::NotConnected,
+        |state| async move {
+            match state {
+                State::NotConnected => match connect().await {
+                    Ok((connection, stream)) => {
+                        (Event::Connected(connection), State::Connected(stream))
+                    }
+                    Err(err) => (Event::Error(err.to_string()), State::Failed),
+                },
+                State::Connected(mut stream) => match stream.next().await {
+                    Some(event) => (event, State::Connected(stream)),
+                    None => iced::futures::future::pending().await,
+                },
+                State::Failed => iced::futures::future::pending().await,
+            }
+        },
+    )
+}
+
+async fn connect() -> zbus::Result<(zbus::Connection, client::EventStream)> {
+    // Connect to session dbus socket
+    let connection = zbus::Connection::session().await?;
+
+    // Start `StatusNotifierWatcher` service, if there isn't one running already
+    server::create_service(&connection).await?;
+
+    // Connect client and listen for registered/unregistered
+    let stream = client::watch(&connection).await?;
+
+    Ok((connection, stream))
+}