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

[mweatherley]

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.

[naman-agrawal-dev]

I think AnimationGraph is a better name when compared to BlendGraph or BlendTree. After all AnimationGraph is a graph that manages animations. It is a state machine just not a discrete state machines. In my opinion there is no change required.

[tim-blackbird]

It is very likely that we will build a discrete animation state machine on top of this API, which would be better suited to the generic AnimationGraph name.

[NthTensor]

Neither AnimationGraph nor BlendGraph really capture what's going on here, but I think BlendGraph is closer to the truth and makes the api level (eg. low) clearer.

[ricky26]

Maybe something like PoseGraph or RenderGraph, though I don't really like either? It's a little bit too graph-like to be what I'd consider a blend tree (maybe that means BlendGraph is good).

I was initially confused because I thought you were considering composing the current graph as the states of the discrete graph, but after talking to @NthTensor, I guess you're thinking about having one graph for "what current pose do I have this frame?" and one for "how does my current state progress over time", then having the latter drive parts of the former.

[BenjaminBrienen]

Are we also renaming Animatable? AnimationDiGraph? AnimatableProperty? AnimationCurve? ...