[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.
Here (and a few other places) you warn about "low level API - use FILE_CLOSE in most cases", can you provide a little more detail about when you would use one versus the other? I think that's useful info.