Fix paste after closing the source

Many popular Ubuntu apps have the problem that Copy/Paste doesn't work if the source is closed before the paste. The current right way of implementing the fix is to make apps conform to the ClipboardManager spec. It's currently being worked on as a Google Summer of Code project and I'd like to investigate alternative ways to solve it, though, because the issue exists in more Ubuntu apps than not, and that fix would require one patch per application.