Generic-less animation framework

[djeedai]

This change rewrites most of the core animation handling to remove generics. Details are listed in the CHANGELOG. See the new Migration Guide to v0.14 for more details

The most notable changes are fixing long-standing user requests or issues:

• No more generics; all Tweenable animations are stepped together with a single system.
• Component-based animation with TweenAnim; spawn the component to start the animation,
  despawning it stops the animation.
• Default auto-despawn on animation completion. (how to remove animator from entity #121, add remove_on_completed flag and new_self_removing fn to Animator and AssetAnimator #122)
• Component-based target with AnimTarget (explicit) or the component of the current Entity (implicit).
  This means you can now host an animation on one Entity, and animate another different Entity
• Event raised at end of each cycle (CycleCompletedEvent) and at end of animation (AnimationCompletedEvent). (A completed event on the Sequence instead of the one on each individual Tween #134)
• Sequence supports repeat count and infinite child animation. (Make Sequence loopable #16, Sequence with repeat_count #79)
• Removes any mention of "progress"; everything is Duration-based. (Get rid of progress and times_completed (well keep an ext trait with it around) in favor of elapsed #31)

Fixes #16
Fixes #31
Fixes #79
Fixes #121
Fixes #134