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/event_loop/window_target.rs

@@ -4,8 +4,10 @@ use crate::event::{DeviceId, ElementState, Event, KeyboardInput, TouchPhase, Win
 use crate::event_loop::ControlFlow;
 use crate::monitor::MonitorHandle as RootMH;
 use crate::window::{Theme, WindowId};
+use std::cell::RefCell;
 use std::clone::Clone;
 use std::collections::{vec_deque::IntoIter as VecDequeIter, VecDeque};
+use std::rc::Rc;
 
 pub struct WindowTarget<T: 'static> {
     pub(crate) runner: runner::Shared<T>,
@@ -42,11 +44,12 @@ impl<T> WindowTarget<T> {
         window::Id(self.runner.generate_id())
     }
 
-    pub fn register(&self, canvas: &mut backend::Canvas, id: window::Id) {
-        let runner = self.runner.clone();
+    pub fn register(&self, canvas: &Rc<RefCell<backend::Canvas>>, id: window::Id) {
+        self.runner.add_canvas(WindowId(id), canvas);
+        let mut canvas = canvas.borrow_mut();
         canvas.set_attribute("data-raw-handle", &id.0.to_string());
-        runner.add_canvas(WindowId(id), canvas.raw().clone());
 
+        let runner = self.runner.clone();
         canvas.on_blur(move || {
             runner.send_event(Event::WindowEvent {
                 window_id: WindowId(id),