Create separate class for CharacterAnimation

This new class copies the animation code specified within
CharacterPreview. Separating the animation was inevitable, as it will
later be reused during the Battle State.
In addition, migrating the animation code in CharacterPreview to this
class will make CharacterPreview much more compact.
Migrate animation code to CharacterAnimation
CharacterAnimation: Reset animation when changing
Remove documentation on migrated attributes

They now reside in CharacterAnimation.