Handle scale factor change on web-sys backend (#1690)

* Change web backend `event_handler` to without  'static lifetime

* Refactor web runner and fix ControlFlow::Exit not being sticky

* Impl. scaling change event for web-sys backend

* Improve `dpi` docs regarding the web backend

* Add changes to changelog

* Update features.md

src/platform_impl/web/event_loop/mod.rs

@@ -28,8 +28,7 @@ impl<T> EventLoop<T> {
 
     pub fn run<F>(self, mut event_handler: F) -> !
     where
-        F: 'static
-            + FnMut(Event<'static, T>, &root::EventLoopWindowTarget<T>, &mut root::ControlFlow),
+        F: 'static + FnMut(Event<'_, T>, &root::EventLoopWindowTarget<T>, &mut root::ControlFlow),
     {
         let target = root::EventLoopWindowTarget {
             p: self.elw.p.clone(),