feat!: The SpriteSheetAnimation is now an asset

[jcornaz]

This is a breaking change!

The SpriteSheetAnimation is now an asset. Which means, one has to insert a Handle<SpriteSheetAnimation> created via Assets::<SpriteSheetAnimation>::add instead of using the SpriteSheetAnimation directly as a component. See the examples for more details.

• Now the animation can be created once and reused many times. Even with some different sprite-sheets, as long as they share the same frame indices, durations and animation mode (once or repeat).
• It is now easier to check what is the current animation as the Handle is a unique identifier of the animation.
• Current animation is now easier (and cheaper) to change, as we no longer need to create a new animation each time. (look at the change_animation for an examble)

It also opens the door for storing animations ar ron (or equivalent) and load them using the asset server, bringing hot-reload support out-of-the-box. (But that's not part of this PR)

Resolve #2

Remaining tasks

• Log an error when SpriteSheetAnimation is used as a component.