Mac: QClipboard behavior different from windows

In some situation where lazy copying is required, On mac there is no option available to set this to happen.

On Windows copy on demand can be implemented with QClipboard by creating a custom QMimeData based class where the protected function retrieveData is called on paste.

On OS X the default behavior for data in the Pasteboard is "EagerRequest", which calls retrieveData immediately.

There does not seems to be a way to set this DataRequestType EagerRequest to LazyRequest without modifying src.

Attached is the little patch which makes the default behavior lazyRequest. But this would be nice if there is way where developer can set what kind of copy he intends to do.