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.

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.

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.

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.