Animation cleanup

[tarcila]

Update the animation system so:

• Animation is decoupled from Scene. A Scene is a static snapshot of a single timestep
• SceneAnimation is the entity in charge of animating/updating a Scene
• Both are being held be the main Context class

Next steps:

• Extract camera animation from the context. The camera animation itself is already handled by the animation system. The context should only need to know the active camera.
• LayerNodeRef index change on reload. Can break transform animation mapping. To be investigated.
• When loading a camera with -usd breaks what's happening with viewport camera manipulations. Not sure why. Happening on next release already and needs to be investigated as a follow up.
• Do another pass on importers where things can be simplified a bit.
• Defrag callback is not implemented for SceneAnimation. It should be implemented by SceneAnimation to update its animations.
• Move to DataTree instead of custom TimeSamples

[jeffamstutz]

LGTM, thanks!