Introduces basic concepts (timelines, alphas, frames)
common to different parts of the Clutter animation API.
Gives a high level overview of the three different
approaches to animation (implicit, ClutterAnimator,
ClutterState).
Added a new recipe (based on the skeleton in the
animations section of the cookbook) about inverting
an animation by reversing the direction of its timeline.
Uses clutter_actor_animate() as the basic approach,
but mentions ClutterState and ClutterAnimator as well.
