Preview view improvements

- [ ] Define an animation sequence. Could have a `Record` button to capture the timing between animations, then `Play`.
- [ ] Save a list of recordings.
- [ ] Allow a recording to be exported.
- [ ] Allow the timeline to be scrubbed. What are the use cases? Per track or all tracks? AnimationState doesn't support reversal -- we would need a recording (an entire animation sequence) so we could start at the beginning and increment to the desired position. Without a recording we can't go backward and the best we can do is a `Next Frame` button, which just increments time 1s / fps (could be useful with speed 0).
- [x] Currently settings (loop, alpha, additive) update the current animation. This makes it impossible to set a looping animation, then set one that doesn't loop. The settings should be for the next animation that plays and not affect the current animation. This makes alpha less useful though, since it can't be changed after setting the animation.
- [ ] Show more information about mixing, like Skeleton Viewer.
- [ ] Show wireframe. When working on relatively flat colored meshes, it can sometimes be a little tricky to see how a mesh is deforming.