This ensures we don't run into discontinuities when reversing an
animation halfway through. I refactored AnimationValue to have knowledge
of the reverse curves and intervals.

This refactors Checkbox to own a RenderObject similar to how Switch was
refactored in https://github.com/domokit/sky_engine/pull/376 and
extracts common functionality for toggleable renderers into a base
class.  Switch and Checkbox's render objects derive from this base
class to add their own custom painting and theming logic.