[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.
Is there code elsewhere that ensures newly-created panes also get this class applied to them?  Might want to add a note here explaining where that happens.

If not (i.e. there's currently a bug) we should consider cleaning this up a bit.  Looking at the LESS code, it appears that this style actually only needs to get applied to the CodeMirror div, _not_ to its parent -- that was just a convenience in the original code to avoid having to loop over all editors.  Now that we're looping over all editors anyway, we might as well just add/remove the class directly there, and not bother fiddling with the holders (panes).