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.

examples/control_flow.rs

@@ -43,8 +43,7 @@ fn main() -> Result<(), impl std::error::Error> {
 
     let event_loop = EventLoop::new().unwrap();
 
-    let mut app = ControlFlowDemo::default();
-    event_loop.run_app(&mut app)
+    event_loop.run_app(ControlFlowDemo::default())
 }
 
 #[derive(Default)]