Implement support for multiple sprites per frame in animations

[rozniak]

There is a requirement that multiple separate sprites (on the same atlas) be used during individual frames of an animation. This issue to track progress on that feature.

Shouldn't be too hard, just extending the JSON format and ActorAnimationFrame.

[rozniak]

I have begun work on this - support exists in the models/code in the repo (under the feat-16 branch) now, it just needs testing. I will update the data in Junkbot as part of rozniak/Junkbot#18 to test the code works as intended.

[rozniak]

Also for clarity, mentions of 'Actor' have been removed from the class names. The classes simply animate sprites and are generic, as a result I have renamed them to the form SpriteAnimation, SpriteAnimationStore etc.

The namespace is now Oddmatics.Rzxe.Game.Animation as well.