Make `Delay::new()` panic if a zero duration is passed as argument. Fix
the `menu` example to skip inserting a `Sequence<Transform>` containing
a zero-duration `Delay`.

Bug: #41

This reverts commit f7647405.

This reverts commit 971edb62.

This reverts commit 87ac60b2.

Upgrade to the latest released version 0.8.0 of Bevy.

Add a new `bevy_asset` feature to enable animation of Bevy assets
(types implementing the `Asset` trait). Bevy 0.8 does not contain
`bevy_asset` in its defaul features, so this new feature reflects that
new possibility to build Bevy and í½ƒ Bevy Tweening without support
for the `bevy_asset` crate. The new feature is enabled by default for
discoverability and to prevent a behavior breaking change.

This reverts commit 6a871576.

This reverts commit 77b56f44.

This reverts commit 722b62ec.

Remove `TweeningType` and split its functionalities between a new `RepeatCount`
controlling the number of repeats of an animation on one hand, and
`RepeatStrategy` controlling the way an animation restarts after a loop ended
on the other hand. This allows more granular control on the type of playback.

Remove the `tweening_type` parameter from `Tween<T>::new()` and replace it with
builder methods `with_repeat_count()` and `with_repeat_strategy()`.

Remove `is_looping()` from all tweenables, which was not implemented for most
of them anyway.

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.

This lets you not have to run after every single system and instead just pass in the label.

It needs to be built-in because otherwise you have to completely recreate the animation to change the speed.

Allow a `Tween` to play backward by setting its direction with
`Tween::set_direction()`.

Change the example in the `lib.rs` module documentation to not rely on
the `bevy_sprite` feature, to allow building cleanly with the
`--no-default-features` flag. Replace the use of `SpriteBundle` with
`TransformBundle`, which is not behind a feature flag so is alway
available.

Enable minimal dependencies with new features `bevu_sprite` and
`bevy_ui`, removing the `bevy/render` mandatory dependency. Those new
features are enabled by default, and enable the built-in lenses for the
related Bevy crates. The core `bevy_tweening` crate itself does not take
any optional Bevy dependency anymore, allowing for a slim build with
only the core Bevy functionalities.

Update all links to Bevy docs to point to v0.6.1, and add to `README.md`
the missing predefined rotation lenses.

Add more predefined rotation lenses interpolating the angle of rotation
instead of the `Quat` itself directly. Document at the `lens` module
level the difference between the shortest-path lens and the
angle-focused ones.

Bug: #5

Fix some bug lerp'ing colored lenses (`TextColorLens`,
`SpriteColorLens`, `ColorMaterialColorLens`) where the alpha value would
be too high. This only affects non-opaque colors, in that for opaque
blending the resulting alpha was `3.0` which was interpreted as opaque.

- 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.

Provide the callback owner with the `Entity` the tween and the animator
are attached to, as a parameter to the callback when invoked.

Add some new methods and refactor some existing ones to clarify the
playback state of a tweenable, and give increased control to the
`Animator` or `AssetAnimator` to rewind a tweenable, set its progress to
an arbitrary value, or query its current state.

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

Ensure the `TweenState::Ended` is set only for `TweeningType::Once`,
which is the only one with a finite duration, so which can end. Other
tweening types repeat indefinitely until stopped, so have no end.