Macos multi windows leak (#481)

* adding a multiwindow example

* Added NSAutoReleasepool for WindowDelegate::Drop
as setDelegate:nil autoreleases WindowDelegate during work.

Added NSAutoReleasepool for Window2::Create,
as it uses autorelease on objects while doing work.

Added NSAutoreleasepool for Window2::Drop
as nswindow::close uses autorelease on objects.

Added NSAutoreleasepool for IdRef.

Moved Window2 WinitWindow objc class to a static var, as we are creating
multiple windows.

* specifying return type for msg_send!

* removing example/recreate_window_leak.rs

* EventLoop, Shared, no need to retain dead weak ptr

* Change log entry added

* added comment about Shared.find_and_remove_window

* fixed code style errors

src/platform/macos/events_loop.rs

@@ -94,7 +94,7 @@ impl Shared {
         if let Ok(mut windows) = self.windows.lock() {
             windows.retain(|w| match w.upgrade() {
                 Some(w) => w.id() != id,
-                None => true,
+                None => false,
             });
         }
     }