Ensure change detection on components and assets is only triggered when
an animator effectively modifies said component or asset, and not
invariably just by the simple fact of the animator ticking each frame.

This change modifies the signature of the `component_animator_system()`
and `asset_animator_system()` public functions to directly consume a
`ResMut<Events<TweenCompleted>>` instead of an
`EventWriter<TweenCompleted>`, to work around some internal limitations.

It also publicly exposes a new `Targetable` trait used to work around
the impossibility to retrieve a `Mut<T>` from a `Mut<Assets<T>>`.
Instead, the trait provides an "target dereferencing" method
`target_mut()` which dereferences the target component or asset, and
triggers its change detection. The trait is implemented for all
components via the `ComponentTarget` object, and for all assets via the
`AssetTarget` object. The 3 types described here are publicly exposed to
workaround some Bevy limitations, and because the trait appears in the
public `Tweenable<T>` API. However users are discouraged from taking
strong dependencies on those, as they will be removed once Bevy provides
a way to achieve this conditionaly dereferencing without this
workaround.

Fixes #33

Add a new `bevy_text` feature for more granular control over
`Text`-related lenses, and change the `TextColorLens` to require that
new feature instead of `bevy_ui`, as texts _can_ be used without the
Bevy UI crate.

Fixes #68

Add two new methods to the `Tweenable<T>` trait:
- `set_elapsed(Duration)` sets the elapsed time of the tweenable. This
  is equivalent to `set_progress(duration().mul_f32(progress))`, with
  the added benefit of avoiding floating-point conversions and potential
  rounding errors resulting from it. `set_progress()` is now largely
  implemented in terms of and as a convenience wrapper to calling
  `set_elapsed()`.
- `elapsed()` which queries the elapsed time of the tweenable. This is
  equivalent to `duration().mul_f32(progress())`, again with better
  precision.

The elapsed API is the recommended way going forward to manipulate a
tweenable's time outside of the normal `tick()` flow. It supersedes the
progress API.

This change purposedly skips any discussion about what happens to
completion events when `set_progress()` or now `set_elpased()` is called
with a lower value than the current one to seek time back. This design
issue is logged as #60 and is still pending. The change also partially
addresses #31, but without removing any existing API or feature.

Clarify testing code and potential assertions message with the use of a
new `assert_approx_eq!()` macro for `f32` equality check with a
tolerance. The macro leverages `abs_diff_eq()` but produces a better
assertion message in case of failure. It also allows skipping the
tolerance parameter to use the default of `1e-5`, which is the
"standard" tolerance to use for progress and other small-ish values that
are expected to be equal but might be slightly off due to rounding
errors.

This change ignores the complications of testing for floating-point
equality in a generic way, which is too complex, and instead restrict
the usage to values like progress (range [0:1]) and other small position
values around the origin.

Add some tests to fill gaps for:
- `UiPositionLens`
- `Animator::set_tweenable()` and `AssetAnimator::set_tweenable()`
- `Sequence::from_single()`
- `Delay`'s `then()`, `times_completed()`, and `set_progress()`

Fix the animator speed applying, which got broken as part of the
refactor of #44.

Add a `speed()` getter to both `Animator<T>` and `AssetAnimator<T>`.

Add some simple test for speed, but this is not enough to make sure the
feature doesn't regress, so logged #62 to follow-up with a proper
regression test.

Fixes #61

Fix the repeat mode being broken after the first iteration due to
`AnimClock::progress()` reporting a progress greater than `1.`, which was
breaking the logic of `Tween` and `Lens`.

Also fix `Tween::rewind()` not restoring the original tween direction when
using a repeat strategy of `RepeatStrategy::MirroredRepeat`.

Fixes #42

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.