Signed-off-by: Alex Saveau <saveau.alexandre@gmail.com>

Fix double-boxing by removing the `IntoBoxDynTweenable` trait and the impl of
`Tweenable<T>` for `Box<dyn Tweenable>`, and instead using some `From`
conversion implemented per concrete type.

Upgrade the version of `bevy-inspector-egui` to `0.10` for Bevy v0.7
compatibility, otherwise the examples were building with Bevy v0.6. Fix
the examples which now build correctly with Bevy v0.7.

- Add `TweenCompleted::user_data` to help identify the tween which
  raised the event.
- Improved several docs.
- Update CHANGELOG.
- Update README with mention of events and callbacks.

Add a mechanism to `Tween<T>` to raise a Bevy event once the tween
completed. The event raising can be enabled with `set_completed_event()`
and `with_completed_event()`. The `TweenCompleted` event contains the
`Entity` on which the `Animator<T>` or `AssetAnimator<T>` owning the
tween are attached.

Update the `sequence` example to show how to use that new event
mechanism.

This avoids having those predefined types, which are optional, be in the
middle of the core types.

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.

- 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