example: Add modal example. (#625)

```
cargo run --example modal
```

crates/story/examples/modal.rs

@@ -0,0 +1,35 @@
+use gpui::*;
+use story::{Assets, ModalStory};
+
+pub struct Example {
+    root: Entity<ModalStory>,
+}
+
+impl Example {
+    pub fn new(window: &mut Window, cx: &mut Context<Self>) -> Self {
+        let root = ModalStory::view(window, cx);
+
+        Self { root }
+    }
+
+    fn view(window: &mut Window, cx: &mut App) -> Entity<Self> {
+        cx.new(|cx| Self::new(window, cx))
+    }
+}
+
+impl Render for Example {
+    fn render(&mut self, _window: &mut Window, _cx: &mut Context<Self>) -> impl IntoElement {
+        div().p_4().size_full().child(self.root.clone())
+    }
+}
+
+fn main() {
+    let app = Application::new().with_assets(Assets);
+
+    app.run(move |cx| {
+        story::init(cx);
+        cx.activate(true);
+
+        story::create_new_window("Modal Example", Example::view, cx);
+    });
+}

crates/story/src/lib.rs

@@ -177,11 +177,18 @@ impl StoryRoot {
 }
 
 impl Render for StoryRoot {
-    fn render(&mut self, _window: &mut Window, _cx: &mut Context<Self>) -> impl IntoElement {
+    fn render(&mut self, window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
+        let drawer_layer = Root::render_drawer_layer(window, cx);
+        let modal_layer = Root::render_modal_layer(window, cx);
+        let notification_layer = Root::render_notification_layer(window, cx);
+
         v_flex()
             .size_full()
             .child(self.title_bar.clone())
             .child(self.view.clone())
+            .children(drawer_layer)
+            .children(modal_layer)
+            .child(div().absolute().top_8().children(notification_layer))
     }
 }