Add methods to set ControlFlow operation

This allows downstream users to avoid importing `ControlFlow` from winit.

examples/request_redraw.rs

@@ -1,7 +1,7 @@
 use simple_logger::SimpleLogger;
 use winit::{
     event::{ElementState, Event, WindowEvent},
-    event_loop::{ControlFlow, EventLoop},
+    event_loop::EventLoop,
     window::WindowBuilder,
 };
 
@@ -17,11 +17,11 @@ fn main() {
     event_loop.run(move |event, _, control_flow| {
         println!("{:?}", event);
 
-        *control_flow = ControlFlow::Wait;
+        control_flow.set_wait();
 
         match event {
             Event::WindowEvent { event, .. } => match event {
-                WindowEvent::CloseRequested => *control_flow = ControlFlow::Exit,
+                WindowEvent::CloseRequested => control_flow.set_exit(),
                 WindowEvent::MouseInput {
                     state: ElementState::Released,
                     ..