Rename AnimationGraph

[BenjaminBrienen]

Objective

Fixes #15604

Solution

Replace "AnimationGraph" with "BlendGraph". Rename "animatable" to "blendable" accordingly.

TODO: Update the comments. I see a lot of "// animation graph". I'll do this once this is blessed and the scope has been determined.

Testing

Simple refactor can be evaluated by CI

Migration Guide

I'll create this once this has been blessed.

[BenjaminBrienen]

The scope of this task needs to be precisely defined. What other types should be renamed for clarity? Should the term "blend" be used in documentation as well?

[hukasu]

Shouldn't the name of the file also be renamed from animatable.rs to blendable.rs?

And what about the extension of the AnimationGraphAssetLoader from .animgraph to .blendgraph?

[BenjaminBrienen]

Good ideas, thanks.

[pcwalton]

I don't have any problem with this.

[mweatherley]

I'm fine with this, but there are still a bunch of places in animatable.rs and animation_curves.rs where "animatable" should be changed to "blendable" (including the name of the former module, probably).

[alice-i-cecile]

Marking as Adopt-Me: I still think this is a good change (and there's support from animation experts), but there's more work to do here and a boatload of merge conflicts.

[ChristopherBiscardi]

adoption attempt in #22782 so closing this one