Add cleanup code to web backend, mostly web-sys (#1715)

* web-sys: Impl. event listeners removal for canvas

* web-sys: Impl. media query listeners cleanup

* web: Emit WindowEvent::Destroyed after Window is dropped

* web-sys: Fix unload event closure being dropped early

* web: Impl. cleanup on ControlFlow::Exit

- Drops the Runner, which causes the event handler closure to be
  dropped.
- (web-sys only:) Remove event listeners from DOM.

* web: Do not remove canvas from DOM when dropping Window

The canvas was inserted by the user, so it should be up to the user
whether the canvas should be removed.

* Update changelog

src/platform_impl/web/web_sys/event_handle.rs

@@ -0,0 +1,65 @@
+use wasm_bindgen::{prelude::Closure, JsCast};
+use web_sys::{AddEventListenerOptions, EventTarget};
+
+pub(super) struct EventListenerHandle<T: ?Sized> {
+    target: EventTarget,
+    event_type: &'static str,
+    listener: Closure<T>,
+}
+
+impl<T: ?Sized> EventListenerHandle<T> {
+    pub fn new<U>(target: &U, event_type: &'static str, listener: Closure<T>) -> Self
+    where
+        U: Clone + Into<EventTarget>,
+    {
+        let target = target.clone().into();
+        target
+            .add_event_listener_with_callback(event_type, listener.as_ref().unchecked_ref())
+            .expect("Failed to add event listener");
+        EventListenerHandle {
+            target,
+            event_type,
+            listener,
+        }
+    }
+
+    pub fn with_options<U>(
+        target: &U,
+        event_type: &'static str,
+        listener: Closure<T>,
+        options: &AddEventListenerOptions,
+    ) -> Self
+    where
+        U: Clone + Into<EventTarget>,
+    {
+        let target = target.clone().into();
+        target
+            .add_event_listener_with_callback_and_add_event_listener_options(
+                event_type,
+                listener.as_ref().unchecked_ref(),
+                options,
+            )
+            .expect("Failed to add event listener");
+        EventListenerHandle {
+            target,
+            event_type,
+            listener,
+        }
+    }
+}
+
+impl<T: ?Sized> Drop for EventListenerHandle<T> {
+    fn drop(&mut self) {
+        self.target
+            .remove_event_listener_with_callback(
+                self.event_type,
+                self.listener.as_ref().unchecked_ref(),
+            )
+            .unwrap_or_else(|e| {
+                web_sys::console::error_2(
+                    &format!("Error removing event listener {}", self.event_type).into(),
+                    &e,
+                )
+            });
+    }
+}