[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.
Find implies that it will return the name of the Document. I can see that `findInPaneViewListMRUOrder()` was migrated from `DocumentManager.findInWorkingSetMRUOrder()`, but since we're changing the name, I think something like `getPaneViewListMRUOrderIndex()` would be a better name for a function that returns an index. What do you think?

If you agree, also rename `findInPaneViewListAddedOrder()` to `getPaneViewListAddedOrderIndex()`.