Auto merge of #455 - xiaopengli89:fix-window-number-at-point, r=jrmuizel

fix: windowNumberAtPoint_belowWindowWithWindowNumber_

[windowNumber(at:belowWindowWithWindowNumber:)](https://developer.apple.com/documentation/appkit/nswindow/1419210-windownumber) is a type method.

Author: bors-servo

cocoa/src/appkit.rs

@@ -1049,7 +1049,7 @@ pub trait NSWindow: Sized {
     unsafe fn setIgnoresMouseEvents_(self, ignoreMouseEvents: BOOL);
     unsafe fn mouseLocationOutsideOfEventStream(self) -> NSPoint;
     unsafe fn setAcceptsMouseMovedEvents_(self, acceptMouseMovedEvents: BOOL);
-    unsafe fn windowNumberAtPoint_belowWindowWithWindowNumber_(self,
+    unsafe fn windowNumberAtPoint_belowWindowWithWindowNumber_(_: Self,
                                                                point: NSPoint,
                                                                windowNumber: NSInteger) -> NSInteger;
 
@@ -1618,10 +1618,10 @@ impl NSWindow for id {
         msg_send![self, setAcceptsMouseMovedEvents:acceptMouseMovedEvents]
     }
 
-    unsafe fn windowNumberAtPoint_belowWindowWithWindowNumber_(self,
+    unsafe fn windowNumberAtPoint_belowWindowWithWindowNumber_(_: Self,
                                                                point: NSPoint,
                                                                windowNumber: NSInteger) -> NSInteger {
-        msg_send![self, windowNumberAtPoint:point belowWindowWithWindowNumber:windowNumber]
+        msg_send![class!(NSWindow), windowNumberAtPoint:point belowWindowWithWindowNumber:windowNumber]
     }
 
     // Converting Coordinates