[REVIEW ONLY] Split View Refactoring

This is the refactoring effort to move Working Set API's and start deprecating some of the DocumentManager APIs

This probably isn't ready for prime time consumption but want to get any feedback on the approach and if there is anything I forgot. 

Also I encourage as much test exposure as possible to make sure nothing is broken.
I don't think you need this explanation of how Array `sort()` compare functions work -- it's pretty standard and you provide a link to Documentation, anyway. This is more appropriate on any implementations than here (where it's passed as a parameter). Also, you can use `@see` so hyperlink is created in API doc.