Expose WindowDestroyed events (#9016)

# Objective

I'm creating an iOS game and had to find a way to persist game state
when the application is terminated. This required listening to the
[`applicationWillTerminate()`
method](https://developer.apple.com/documentation/uikit/uiapplicationdelegate/1623111-applicationwillterminate),
but I cannot do so myself anymore since `winit` already set up a
delegate to listen for it, and there can be only one delegate.

So I had to move up the stack and try to respond to one of the events
from `winit` instead. It appears `winit` fires two events that could
serve my purpose: `WindowEvent::Destroyed` and `Event::LoopDestroyed`.
It seemed to me the former might be slightly more generally useful, and
I also found a past discussion that suggested it would be appropriate
for Bevy to have a `WindowDestroyed` event:
bevyengine/bevy#5589 (comment)

## Solution

- I've added the `WindowDestroyed` event, which fires when `winit` fires
`WindowEvent::Destroyed`.

---

## Changelog

### Added

- Introduced a new `WindowDestroyed` event type. It is used to indicate
a window has been destroyed by the windowing system.

src/lib.rs

@@ -41,8 +41,8 @@ use bevy_utils::{
 use bevy_window::{
     exit_on_all_closed, CursorEntered, CursorLeft, CursorMoved, FileDragAndDrop, Ime,
     ReceivedCharacter, RequestRedraw, Window, WindowBackendScaleFactorChanged,
-    WindowCloseRequested, WindowCreated, WindowFocused, WindowMoved, WindowResized,
-    WindowScaleFactorChanged, WindowThemeChanged,
+    WindowCloseRequested, WindowCreated, WindowDestroyed, WindowFocused, WindowMoved,
+    WindowResized, WindowScaleFactorChanged, WindowThemeChanged,
 };
 
 #[cfg(target_os = "android")]
@@ -231,6 +231,7 @@ struct WindowEvents<'w> {
     window_focused: EventWriter<'w, WindowFocused>,
     window_moved: EventWriter<'w, WindowMoved>,
     window_theme_changed: EventWriter<'w, WindowThemeChanged>,
+    window_destroyed: EventWriter<'w, WindowDestroyed>,
 }
 
 #[derive(SystemParam)]
@@ -638,6 +639,11 @@ pub fn winit_runner(mut app: App) {
                             theme: convert_winit_theme(theme),
                         });
                     }
+                    WindowEvent::Destroyed => {
+                        window_events.window_destroyed.send(WindowDestroyed {
+                            window: window_entity,
+                        });
+                    }
                     _ => {}
                 }