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/stdweb/scaling.rs

@@ -0,0 +1,13 @@
+use super::super::ScaleChangeArgs;
+
+pub struct ScaleChangeDetector(());
+
+impl ScaleChangeDetector {
+    pub(crate) fn new<F>(_handler: F) -> Self
+    where
+        F: 'static + FnMut(ScaleChangeArgs),
+    {
+        // TODO: Stub, unimplemented (see web_sys for reference).
+        Self(())
+    }
+}