[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.
The docs here need to make it clear that this is only the list of files in the pane's _working list_ -- there may be one more file "open" in the pane which will not be returned in this list.

Similarly, `getAllOpenFiles()` below should explain that it _does_ include those extra files that may not be added to the list yet.

In general I think we should scrub the API docs (and maybe API naming -- e.g. maybe this should actually be `getWorkingFiles()`) to make sure that distinction is very crisp -- otherwise we'll see a bunch of extensions misusing these APIs, possibly breaking whenever the user has a file open that's not added to the list yet.