Rename `AnimationGraph` to something like `BlendGraph` or `BlendTree`

From time to time, someone gets confused about what the `AnimationGraph` actually does. Its name is probably too easily construed as something that might resemble a state machine, when all it does is blend animation clips. A name that reflects that is probably better-suited.

Start by locating the AnimationGraph definition and all references to its public name. Review the discussion and compare the proposed BlendGraph and BlendTree names before determining the intended scope. Done means the project has an agreed replacement name and all affected references are updated consistently.

Written by the indexing model from the issue text.