Animate layout changes

[natinusala]

It would be nice to have a way to animate layout changes. Instead of having the layout pass done and every view be resized at once, the changes would be interpolated and the whole UI would smoothly "move" from the old layout to the new one.

For example, collapsing a sidebar would expand the main aera on the right. The eShop on the Switch does it, for a visual example. Another example, changing the text of a button would increase its width smoothly.

Every layout change should be animatable to have a smooth transition.

Probably related to #150 ?