[Feature] Transitioning between pages using fade in/out animation

Hi Andrea,
I really loved this `bottomNavigationBar` implementation because it persists the states of the pages.

I tried myself very hard but could not get fade in/out animations while changing `Offstage` page one to another to work.

I believe it would look so much nicer to have a very quick and simple animation transitioning between the pages.

Also while looking at `Offstage`'s documentation it says:

> /// * [Visibility], which can hide a child more efficiently (albeit less
> /// subtly).
> /// * [TickerMode], which can be used to disable animations in a subtree.

Do you think `Offstage` would not be performant if our navigation stacks are deep?

Thanks.