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 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 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.

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.

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

- 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.

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 `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