Rewrite TextureAtlas system to support Json files

Using the Json Serializer classes, TextureAtlas has been rewritten to support simple Json formatted files.  It's still capable of loading legacy atlas files.

With a little bit of work from someone who knows the TexturePacker2 app, we could probably use the write methods from Json.Serializer to export new TextureAtlases as json files.

Most data types such as pages and regions have been slightly simplified and are private to the class.  TextureAtlas also now no longer returns AtlasSprites and AtlasRegions.