SplitView Refactoring

This replaces #8045 

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'm ok with `--index`. I just didn't look at code closely enough. No need for any changes.