Fixed: CPAlert always shows on main platform window

Previously, a CPAlert was always shown on the main platform window. This behavior could result in a feeling that the application was hanging because of the modal nature of the alert when users are looking to another platform window. This fix ensure to set the CPAlert window's platformWindow to the current key window using [[CPApp keyWindow] platformWindow].

AppKit/CPAlert.j

@@ -642,6 +642,7 @@ var bottomHeight = 71;
 
     _window = [[CPPanel alloc] initWithContentRect:frame styleMask:forceStyle || _defaultWindowStyle];
     [_window setLevel:CPStatusWindowLevel];
+    [_window setPlatformWindow:[[CPApp keyWindow] platformWindow]];
 
     if (_title)
         [_window setTitle:_title];