[REVIEW_ONLY] Splitview

* This pr creates a split view of either side-by-side or upper-lower views of 2 different files.
* From the View Menu select "Split Vertically" or "Split Horizontally" to create a second pane. 
* Selecting the menu item again will collapse the panes down to a single pane (merging the two working sets and showing the current view)
* Selecting the opposing menu item will just reorient the views without disturbing the documents
* A blue outline currently denotes the active pane.  Functions such as opening a document, searching, etc... will target the active pane.  Functions like Close All will target both panes and close all documents.
* Lint will work on the currently focused editor
* Images are not supported and viewing images has been disabled in this build
* The UI as it works now is temporary.  A UI treatment story will come next with @larz0 UI treatments

Known Issue:
https://github.com/adobe/brackets/issues/8513#issuecomment-49796161
The selection is getting reworked with @larz0 UI treatment and will be addressed at that time

I encourage anyone who has cycles to test this.  Obviously this is a big change and needs quite a number of eyes on it.
`waitsForDone()` should be the last thing in a runs() block -- anything added after it might run before the "done" occurs.  In this case I _think_ it will actually always work anyway, because all the files have been opened at least once and thus switching between them is never async... but it's not ideal to structure it this way.

A simple fix would be:
* Change `runCloseOthers()` so it contains a pair of runs() blocks _inside_ it
* The first runs() ends in `waitsForDone()`
* Second runs() contains just this `expect()`
* (Optional) Clean up the usages below to call `runCloseOthers()` directly.  (This is how most of our tests are structured -- I think whoever wrote these was just less familiar with Jasmine).