[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 think it's fine as-is.  `string::find` (C++) returns the index of the occurrence of the sub string so this isn't a new pattern.  It's also been around for awhile and is easier to map the old API to the new.  It's already in documentation that way which has gone through several rounds of debate already.