Add a should_exit method to Application trait

A Cosmic application has no way to call `Cosmic<T>::close` to set the
`should_exit` flag there, as far as I'm aware.

So it makes sense to add a `should_exit` method here.

src/app/cosmic.rs

@@ -104,7 +104,7 @@ where
 
     #[cfg(feature = "wayland")]
     fn should_exit(&self) -> bool {
-        self.should_exit
+        self.should_exit || self.app.should_exit()
     }
 
     fn style(&self) -> <Self::Theme as iced_style::application::StyleSheet>::Style {