Animating child size changes

[makoConstruct]

In the flex widgets, layout often results in size changes in the children. Per shouldAnimateSize (currently nonfunctional) it would be helpful if we could animate those.

[makoConstruct]

The problem is that we want the AnimatedFlex to be responsive to size changes, but in an animated way, without being affected by the size change animations it creates.

If a child triggers a layout change, that should cause us to do a layout. But if we do a layout and then administer animated size changes to the children, we shouldn't be responding to its calls to relayout from that... :/

[makoConstruct]

Actually, maybe we should leave it to the child widgets? There are things child widgets can do, for instance, in the examples, if the background animated, but the content didn't, that would actually be perfect. So, the animation wouldn't need to trigger any layout changes that could percolate back up.
Also consider if you had nested AnimatedWraps, which have that kind of animation style, they would animate size changes just fine.

So I'd say the solution is to provide NimatedContainer and other such widgets so that users don't feel a need to impose animations in this way.

There might be other approaches we could take, but I personally want this one first.

[makoConstruct]

Making a NimatedContainer would entail making a NimatedDecoration Clip and a nimated decoratedbox. It may suffice just to nimate the decoration, since the clipper just calls the decoration.

It may be possible to do this using Decoration::lerpTo and abusing BoxPainter::onChanged

[makoConstruct]

Mostly solved with RanimatedContainer in 5bc3ce3 , but there's now a problem where the circular reveal animation is hanging around and interferes with box size changes: qwert2603/circular_reveal_animation#9

[makoConstruct]

The ranimation approach basically works, just waiting on qwert2603/circular_reveal_animation#10

If you use an insertion animation other than circular reveal, it works.

[makoConstruct]

We'd also like to have the other more obvious way of animating size changes (bool shouldAnimateSize), but I'm not doing that right now.