[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.
I'm not sure that makes sense.  Having it switch to another tab in the middle would be strange IMO.  I'm not sure I have anything to compare it against but this would be somewhat difficult to implement since each pane owns its own MRU list plus there may be some pane activation issues that might cause problems with traversing the MRU list.  Specifically if a pane is activated a few events get sent down the line that someone could do something dangerous in the middle of our traversal.

Actually I just checked visual studio and they show a modeless dialog to show the traversal order.  What would think about doing something like that to avoid issues with Pane Activation in the middle of traversal?

![vs-traversal-dialog](https://cloud.githubusercontent.com/assets/1514099/3680002/bbd4ad7e-12aa-11e4-89f8-1973f1192ea3.png)