[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.
It might be too much to bite off now, but I'd sure like to see this special-casing go away in the future.  Maybe in an ideal world it would look like this:

* EditorManager is the default/fallback provider in MainViewFactory
* Move the `DocumentManager.getDocumentForPath()` call down into that provider, so there's no separate codepath here
* Ensure everything EditorManager needs to do in terms of reparenting editors, refreshing layout when shown, saving scroll-state before hiding, etc. is accessible via the generic View interface
* Maybe even move open() into MainViewFactory and make it the main entry point for people who know what file they want to open but not what type of viewer it needs... or maybe make open() private and have FILE_OPEN be that entry point.

But yeah, no need to undertake that now...