SplitView 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.
Typo: `@retnr`.

Should also mention in a //TODO that `paneId` is not yet used, but will be when multiple panels are supported.