Add methods to set ControlFlow operation

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

examples/transparent.rs

@@ -1,7 +1,7 @@
 use simple_logger::SimpleLogger;
 use winit::{
     event::{Event, WindowEvent},
-    event_loop::{ControlFlow, EventLoop},
+    event_loop::EventLoop,
     window::WindowBuilder,
 };
 
@@ -18,14 +18,14 @@ fn main() {
     window.set_title("A fantastic window!");
 
     event_loop.run(move |event, _, control_flow| {
-        *control_flow = ControlFlow::Wait;
+        control_flow.set_wait();
         println!("{:?}", event);
 
         match event {
             Event::WindowEvent {
                 event: WindowEvent::CloseRequested,
                 ..
-            } => *control_flow = ControlFlow::Exit,
+            } => control_flow.set_exit(),
             _ => (),
         }
     });