This ensures that the only strong reference to the presenter is held
by `App`. This is important because we want to call `flush_effects`
when removing a window and implicit drops of the `Presenter` would
make that hard.

Before this commit, if a rendered view contained strong handles to
views and models, we would only drop them on the next `flush_effects`.
This was manifesting itself in `Project`s not being released when
closing their containing window.