Add methods to set ControlFlow operation

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

examples/custom_events.rs

@@ -3,7 +3,7 @@ fn main() {
     use simple_logger::SimpleLogger;
     use winit::{
         event::{Event, WindowEvent},
-        event_loop::{ControlFlow, EventLoopBuilder},
+        event_loop::EventLoopBuilder,
         window::WindowBuilder,
     };
 
@@ -34,14 +34,14 @@ fn main() {
     });
 
     event_loop.run(move |event, _, control_flow| {
-        *control_flow = ControlFlow::Wait;
+        control_flow.set_wait();
 
         match event {
             Event::UserEvent(event) => println!("user event: {:?}", event),
             Event::WindowEvent {
                 event: WindowEvent::CloseRequested,
                 ..
-            } => *control_flow = ControlFlow::Exit,
+            } => control_flow.set_exit(),
             _ => (),
         }
     });