Need to be able to stop presenters

[ghost]

Presenter#takeView performs a registration that cannot be undone. If a long lived scope needs to have some presenters that come and go, it really can't.

I think that MortarScope#register needs to return a Registration or Subscription object with a void unsubsribe() method. And I think that Presenter needs a void stop() method that makes it undo the registration it sets up in takeView.

[pyricau]

Correct me if I'm wrong, but until now I think we had handled that by creating subscopes. What's changed here?

[ghost]

I'm in a situation where I don't have a subscope. But you have a point there, maybe I should just make one. I'll try that out and close this if it seems practical.

cc @dnkoutso