[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 put a comment here explaining why it's ok to not bail when the doc is already open in some pane?

I think it's because we only get here for cases handled by EditorManager -- not any custom viewers -- and we can trust `EditorManager.openDocument()` to not break things by adding two views of the same file.  And we do still need to call EditorManager, because it needs to do extra work to reparent the editor, if we're asking to show it in a different pane now.*  Is all of that correct?

\* I'm confused by this part, though: is there ever a case where we ask to move an editor from one pane to a different one?  It can't be done by merely calling open() with a different paneId...