Add last window closed setting (#25185)

Release Notes:

- Added an `on_last_window_closed` setting, that allows users to quit
the app when the last window is closed

---------

Co-authored-by: Richard <richard@zed.dev>

crates/gpui/examples/on_window_close_quit.rs

@@ -0,0 +1,82 @@
+use gpui::{
+    actions, div, prelude::*, px, rgb, size, App, Application, Bounds, Context, FocusHandle,
+    KeyBinding, Window, WindowBounds, WindowOptions,
+};
+
+actions!(example, [CloseWindow]);
+
+struct ExampleWindow {
+    focus_handle: FocusHandle,
+}
+
+impl Render for ExampleWindow {
+    fn render(&mut self, _window: &mut Window, _cx: &mut Context<Self>) -> impl IntoElement {
+        div()
+            .on_action(|_: &CloseWindow, window, _| {
+                window.remove_window();
+            })
+            .track_focus(&self.focus_handle)
+            .flex()
+            .flex_col()
+            .gap_3()
+            .bg(rgb(0x505050))
+            .size(px(500.0))
+            .justify_center()
+            .items_center()
+            .shadow_lg()
+            .border_1()
+            .border_color(rgb(0x0000ff))
+            .text_xl()
+            .text_color(rgb(0xffffff))
+            .child(
+                "Closing this window with cmd-w or the traffic lights should quit the application!",
+            )
+    }
+}
+
+fn main() {
+    Application::new().run(|cx: &mut App| {
+        let mut bounds = Bounds::centered(None, size(px(500.), px(500.0)), cx);
+
+        cx.bind_keys([KeyBinding::new("cmd-w", CloseWindow, None)]);
+        cx.on_window_closed(|cx| {
+            if cx.windows().is_empty() {
+                cx.quit();
+            }
+        })
+        .detach();
+
+        cx.open_window(
+            WindowOptions {
+                window_bounds: Some(WindowBounds::Windowed(bounds)),
+                ..Default::default()
+            },
+            |window, cx| {
+                cx.activate(false);
+                cx.new(|cx| {
+                    let focus_handle = cx.focus_handle();
+                    focus_handle.focus(window);
+                    ExampleWindow { focus_handle }
+                })
+            },
+        )
+        .unwrap();
+
+        bounds.origin.x += bounds.size.width;
+
+        cx.open_window(
+            WindowOptions {
+                window_bounds: Some(WindowBounds::Windowed(bounds)),
+                ..Default::default()
+            },
+            |window, cx| {
+                cx.new(|cx| {
+                    let focus_handle = cx.focus_handle();
+                    focus_handle.focus(window);
+                    ExampleWindow { focus_handle }
+                })
+            },
+        )
+        .unwrap();
+    });
+}