Allow closing windows at runtime (#3575)

# Objective

Fixes #3180, builds from bevyengine/bevy#2898

## Solution

Support requesting a window to be closed and closing a window in `bevy_window`, and handle this in `bevy_winit`.

This is a stopgap until we move to windows as entites, which I'm sure I'll get around to eventually.

## Changelog

### Added

- `Window::close` to allow closing windows.
- `WindowClosed` to allow reacting to windows being closed.

### Changed

Replaced `bevy::system::exit_on_esc_system` with `bevy::window::close_on_esc`.

## Fixed

The app no longer exits when any window is closed. This difference is only observable when there are multiple windows. 

## Migration Guide

`bevy::input::system::exit_on_esc_system` has been removed. Use `bevy::window::close_on_esc` instead.
`CloseWindow` has been removed. Use `Window::close` instead.
The `Close` variant has been added to `WindowCommand`. Handle this by closing the relevant window.

src/lib.rs

@@ -2,39 +2,38 @@ mod converters;
 mod winit_config;
 mod winit_windows;
 
-use bevy_input::{
-    keyboard::KeyboardInput,
-    mouse::{MouseButtonInput, MouseMotion, MouseScrollUnit, MouseWheel},
-    touch::TouchInput,
-};
 pub use winit_config::*;
 pub use winit_windows::*;
 
 use bevy_app::{App, AppExit, CoreStage, Plugin};
+use bevy_ecs::prelude::*;
 use bevy_ecs::{
-    event::{EventWriter, Events, ManualEventReader},
-    schedule::ParallelSystemDescriptorCoercion,
-    system::{NonSend, ResMut},
+    event::{Events, ManualEventReader},
     world::World,
 };
+use bevy_input::{
+    keyboard::KeyboardInput,
+    mouse::{MouseButtonInput, MouseMotion, MouseScrollUnit, MouseWheel},
+    touch::TouchInput,
+};
 use bevy_math::{ivec2, DVec2, Vec2};
 use bevy_utils::{
-    tracing::{error, trace, warn},
+    tracing::{error, info, trace, warn},
     Instant,
 };
 use bevy_window::{
     CreateWindow, CursorEntered, CursorLeft, CursorMoved, FileDragAndDrop, ModifiesWindows,
     ReceivedCharacter, RequestRedraw, WindowBackendScaleFactorChanged, WindowCloseRequested,
-    WindowCreated, WindowFocused, WindowMoved, WindowResized, WindowScaleFactorChanged, Windows,
+    WindowClosed, WindowCreated, WindowFocused, WindowMoved, WindowResized,
+    WindowScaleFactorChanged, Windows,
 };
+
 use winit::{
-    dpi::PhysicalPosition,
+    dpi::{LogicalSize, PhysicalPosition},
     event::{self, DeviceEvent, Event, StartCause, WindowEvent},
     event_loop::{ControlFlow, EventLoop, EventLoopWindowTarget},
 };
 
-use winit::dpi::LogicalSize;
-
 #[derive(Default)]
 pub struct WinitPlugin;
 
@@ -51,10 +50,12 @@ impl Plugin for WinitPlugin {
 }
 
 fn change_window(
-    winit_windows: NonSend<WinitWindows>,
+    mut winit_windows: NonSendMut<WinitWindows>,
     mut windows: ResMut<Windows>,
     mut window_dpi_changed_events: EventWriter<WindowScaleFactorChanged>,
+    mut window_close_events: EventWriter<WindowClosed>,
 ) {
+    let mut removed_windows = vec![];
     for bevy_window in windows.iter_mut() {
         let id = bevy_window.id();
         for command in bevy_window.drain_commands() {
@@ -166,9 +167,25 @@ fn change_window(
                         window.set_max_inner_size(Some(max_inner_size));
                     }
                 }
+                bevy_window::WindowCommand::Close => {
+                    // Since we have borrowed `windows` to iterate through them, we can't remove the window from it.
+                    // Add the removal requests to a queue to solve this
+                    removed_windows.push(id);
+                    // No need to run any further commands - this drops the rest of the commands, although the `bevy_window::Window` will be dropped later anyway
+                    break;
+                }
             }
         }
     }
+    if !removed_windows.is_empty() {
+        for id in removed_windows {
+            // Close the OS window. (The `Drop` impl actually closes the window)
+            let _ = winit_windows.remove_window(id);
+            // Clean up our own data structures
+            windows.remove(id);
+            window_close_events.send(WindowClosed { id });
+        }
+    }
 }
 
 fn run<F>(event_loop: EventLoop<()>, event_handler: F) -> !
@@ -316,7 +333,8 @@ pub fn winit_runner_with(mut app: App) {
                 let window = if let Some(window) = windows.get_mut(window_id) {
                     window
                 } else {
-                    warn!("Skipped event for unknown Window Id {:?}", winit_window_id);
+                    // If we're here, this window was previously opened
+                    info!("Skipped event for closed window: {:?}", window_id);
                     return;
                 };
                 winit_state.low_power_event = true;

src/winit_windows.rs

@@ -181,6 +181,12 @@ impl WinitWindows {
     pub fn get_window_id(&self, id: winit::window::WindowId) -> Option<WindowId> {
         self.winit_to_window_id.get(&id).cloned()
     }
+
+    pub fn remove_window(&mut self, id: WindowId) -> Option<winit::window::Window> {
+        let winit_id = self.window_id_to_winit.remove(&id)?;
+        // Don't remove from winit_to_window_id, to track that we used to know about this winit window
+        self.windows.remove(&winit_id)
+    }
 }
 
 pub fn get_fitting_videomode(