WIP - Make poll_events and run_forever take &mut self

This removes the need for the EventsLoop::interrupt method by inroducing
a ControlFlow type. This new type is to be returned by the user's
callback and indicates whether the `EventsLoop` should continue waiting
for events or break from the loop.

Only the wayland, x11 and api_transition backends have been updated so
far, and only the wayland backend has actually been tested.

examples/window.rs

@@ -12,8 +12,10 @@ fn main() {
         println!("{:?}", event);
 
         match event {
-            winit::Event::WindowEvent { event: winit::WindowEvent::Closed, .. } => events_loop.interrupt(),
-            _ => ()
+            winit::Event::WindowEvent { event: winit::WindowEvent::Closed, .. } => {
+                winit::ControlFlow::Complete
+            },
+            _ => winit::ControlFlow::Continue,
         }
     });
 }