The time interval for animations is most often described as `duration`
in animation contexts and the `WindowServer::Animation` class
should reflect that.
This patch adds the WindowServer::Animation class, which represents
a simple animation driven by the compositor.
An animation has a length (in milliseconds) and two hooks:
- on_update: called whenever the animation should render something.
- on_stop: called when the animation is finished and/or stopped.
This patch also ports the window minimization animation to this new
mechanism. :^)