AnimationEvent -> Event and other improvements (bevyengine#16440)

# Objective

Needing to derive `AnimationEvent` for `Event` is unnecessary, and the
trigger logic coupled to it feels like we're coupling "event producer"
logic with the event itself, which feels wrong. It also comes with a
bunch of complexity, which is again unnecessary. We can have the
flexibility of "custom animation event trigger logic" without this
coupling and complexity.

The current `animation_events` example is also needlessly complicated,
due to it needing to work around system ordering issues. The docs
describing it are also slightly wrong. We can make this all a non-issue
by solving the underlying ordering problem.

Related to this, we use the `bevy_animation::Animation` system set to
solve PostUpdate animation order-of-operations issues. If we move this
to bevy_app as part of our "core schedule", we can cut out needless
`bevy_animation` crate dependencies in these instances.

## Solution

- Remove `AnimationEvent`, the derive, and all other infrastructure
associated with it (such as the `bevy_animation/derive` crate)
- Replace all instances of `AnimationEvent` traits with `Event + Clone`
- Store and use functions for custom animation trigger logic (ex:
`clip.add_event_fn()`). For "normal" cases users dont need to think
about this and should use the simpler `clip.add_event()`
- Run the `Animation` system set _before_ updating text
- Move `bevy_animation::Animation` to `bevy_app::Animation`. Remove
unnecessary `bevy_animation` dependency from `bevy_ui`
- Adjust `animation_events` example to use the simpler `clip.add_event`
API, as the workarounds are no longer necessary

This is polishing work that will land in 0.15, and I think it is simple
enough and valuable enough to land in 0.15 with it, in the interest of
making the feature as compelling as possible.

crates/bevy_animation/Cargo.toml

@@ -10,7 +10,6 @@ keywords = ["bevy"]
 
 [dependencies]
 # bevy
-bevy_animation_derive = { path = "derive", version = "0.15.0-dev" }
 bevy_app = { path = "../bevy_app", version = "0.15.0-dev" }
 bevy_asset = { path = "../bevy_asset", version = "0.15.0-dev" }
 bevy_color = { path = "../bevy_color", version = "0.15.0-dev" }