Change run_app(app: &mut A) to run_app(app: A) (#3721)

This allows the user more control over how they pass their application state
to Winit, and will hopefully allow `Drop` implementations on the application
handler to work in the future on all platforms.

src/changelog/unreleased.md

@@ -65,6 +65,9 @@ changelog entry.
   `ApplicationHandler::resumed/suspended()` are now only emitted by iOS and Web
   and now signify actually resuming/suspending the application.
 - Rename `platform::web::*ExtWebSys` to `*ExtWeb`.
+- Change signature of `EventLoop::run_app`, `EventLoopExtPumpEvents::pump_app_events` and
+  `EventLoopExtRunOnDemand::run_app_on_demand` to accept a `impl ApplicationHandler` directly,
+  instead of requiring a `&mut` reference to it.
 
 ### Removed