[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.
Should we say anything about how views are initially created?  E.g.:

* Calling the View constructor is expected to create an orphaned DOM node, and getContainer() should always return that DOM node.
* Pane then immediately adds it to the DOM (and calls notifyContainerChanged() to tell the View what its DOM parent is).
* If the DOM parent ever changes again, notifyContainerChanged() is called again.

I'm not actually sure if it works that way, but something on that level of detail would be good.  Otherwise it's hard to tell how you'd go about implementing a new custom viewer.