Support event subscriptions in iced_web

Also improves the overall web runtime, avoiding nested update loops.

src/application.rs

@@ -233,6 +233,7 @@ where
     A: Application,
 {
     type Message = A::Message;
+    type Executor = A::Executor;
 
     fn new() -> (Self, Command<A::Message>) {
         let (app, command) = A::new();
@@ -248,6 +249,10 @@ where
         self.0.update(message)
     }
 
+    fn subscription(&self) -> Subscription<Self::Message> {
+        self.0.subscription()
+    }
+
     fn view(&mut self) -> Element<'_, Self::Message> {
         self.0.view()
     }