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.

src/window.rs

@@ -110,7 +110,7 @@ impl WindowBuilder {
         }
 
         // building
-        let w = try!(platform::Window2::new(events_loop.events_loop.clone(), &self.window, &self.platform_specific));
+        let w = try!(platform::Window2::new(&events_loop.events_loop, &self.window, &self.platform_specific));
 
         Ok(Window { window: w })
     }