Update the macOS backend to the new futures-compatible API.

This is a follow up to the new API introduced in #20.

This also fixes the issue where window resize events would not be
emitted until the end of the resize. This PR fixese #39 by ensuring that
the user callback given to either `EventsLoop::poll_events` or
`EventsLoop::run_forever` can be called by each window delegate's resize
callback directly.

src/window.rs

@@ -2,7 +2,6 @@ use std::collections::vec_deque::IntoIter as VecDequeIter;
 
 use CreationError;
 use CursorState;
-use WindowEvent as Event;
 use EventsLoop;
 use MouseCursor;
 use Window;