[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.
Drive-by: if an inline editor becomes active, it seems like we should focus the pane that contains the full editor that contains that inline editor. I played around with this in the branch and there doesn't seem to be a bug right now, because the only way you can focus an inline editor is by clicking on it, and the click handler for the pane catches that case currently. (I was able to trigger the bug when I had a breakpoint set, probably because it interfered with the click handler, but can't think of a way for the user to cause it to happen in normal use.) But if there were some other way to cause an inline editor to become focused in the future, it could be a problem.