Update morph_targets and many_foxes examples to use observers

[greeble-dev]

Objective

Change some examples to follow best practices for playing animations, and as a bonus work around an issue with scenes spawning multiple times.

Background

Examples that play skeletal animations usually have two steps:

1. Start scene spawning.
2. Play animations after the scene has spawned.

Different examples use different approaches for triggering part 2, including a scene spawning observer and Added<AnimationPlayer> queries.

The observer approach is arguably best as it's more tightly scoped and easier for users to extend. The other approaches work in simple examples but fall down when users want multiple scenes or animations. See #17421 for more detail.

As a bonus, the scene spawning observer works around a current issue with scenes spawning multiple times - see #20393, #20430. Although there's an argument that this PR shouldn't land until the issue is properly fixed, as these examples are a useful test case.

Solution

Update the morph_targets and many_foxes examples to use observers.

I also made a few tweaks and fixes to morph_targets.

• Fix documentation referring to an update_weights feature that isn't in the example.
• Use the same AnimationToPlay component as the animated_mesh example.
• Change the name_morphs system to be event driven and print the asset name.
  • This is maybe too complex, but could also be nice for users to c&p into their app for debugging.

I haven't updated the animated_mesh_control, animated_mesh_events, and animation_masks examples, which still use Added<AnimationPlayer>.

Testing

cargo run --example morph_targets
cargo run --example many_foxes

[alice-i-cecile]

I think this is an improvement regardless of the asset reloading problem.

[alice-i-cecile]

Waiting to merge until we have a fix for the issue that this revealed.