Support keyframing of pairBlended attributes

Whenever you keyframe a constrained channel, it'll create a pairBlend. If you then subsequently attempt to..

```py
node["translateX"] = {1: 0.0, 5: 10.0, 7: 11.0}
```

Then that new animation will *break* thee pairBlend, rather than get redirected to wherever it's blending.