Add support for animatable size property

[benjaminbojko]

This is tricky and not implemented yet because some views rely on setSize() to set internal flags for the next update run to stay efficient with resizing/recalculating layouts (e.g. TextView). We could do a check on update to see if the size has changed via an animation (since this won't call setSize()) or we could add a method that specifically allows us to animate size (but this would introduce a new paradigm).