*withState() hooks should clear subject state when no longer used

[mkrause]

In the useModalWithSubject and useOverlayWithSubject hooks, the subject state is kept around even after the modal/overlay is closed. It doesn't cause any particular issues, but it does hold on to an object reference that could otherwise be garbage collected.

Would require something like an onUnmount callback in ModalProvider/PopoverProvider so that the consumer gets notified when the unmount happens and the state can be reset.