[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.
We are really inconsistent with the use of `!` and different JSDocs have slightly different documentations.

In https://developers.google.com/closure/compiler/docs/js-for-compiler#types you don't need to add `!` for basic types (functions, numbers, strings, booleans). But in http://usejsdoc.org/tags-type.html there is no mention about that. Since we are going to be more consistent about the JSDocs, it will be worth mentioning the use of `!` and `?` in the parameters in the new wiki page https://github.com/adobe/brackets/wiki/Brackets-JSDocs-Guidelines