Enhancement for HasPopupSlot

As a bug fix for [PresenterWidget: passing null to addToPopupSlot() doesn't clear the popup slot](https://code.google.com/p/gwt-platform/issues/detail?id=442) I propose to enhance the interface ```HasPopupSlot``` with a method ```removeFromPopupSlot(PresenterWidget<? extends PopupView> child)```. Thus the interface would have similar methods as the ```HasSlots``` interface and both implementations could behave in a similar way. Additionally the (IMHO) a little akward behaviour to pass null to an add-method to remove something (which - as stated correctly in the issue - does not work anyway) could be removed.
If there aren't any objections I'd like to give try at the weekend and submit a pull request.