[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've been using bulleted lists:

```
 *    - editorAreaResize -- When editor-holder's size changes for any reason (including panel show/hide
 *          - panel resize, or the window resize).
 *          - The 2nd arg is the new editor-holder height.
 *          - The 3rd arg is a refreshHint flag for internal EditorManager use.
```