Add a `Tweenable<T>` trait describing a generic animatable tween-like
element.

Expose the three types of tweenables and implement `Tweenable<T>` for
them:
- `Tween<T>`, a single animation
- `Sequence<T>`, a sequence of consecutive animations
- `Tracks<T>`, a batch of animations running in parallel

Clean-up animators to hold a single top-level `Tweenable<T>` and let the
user build any kind of animation hierarchy for themselves.

Allow querying the active tween of a sequence. Use this to improve the
`sequence` example with some text showing the current active tween and
its progress.

Implement some callbacks invoked when a tween anim starts or ends.

Export publicly the asset and component system to allow an app to
manually add them.

Add CI

- Add `Sequence<T>` for chained tweens
- Add `Tracks<T>` for tracks of sequences running in parallel
- Move most animation-related properties to the new `Tweens<T>` struct
- Add `sequence` example

- Fix license name and color
- Add Bevy tracking badge

Seems like `bevy_easings` moved too... :)

Import `bevy_tweening` from its old repository.

URL:
https://github.com/djeedai/bevy_extra/tree/main/crates/bevy_tweening

Commit: 1e13a72