[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.
I forget the exact discussion we had about this a few months ago, but it feels a cleaner to me to leave `toggleInlineWidget()` and `openInlineWidget()` in EditorManager.  Searching through a list of providers and finding the one to instantiate doesn't feel like a core function of Editor -- it feels like something we layer on top of it, separably.  Editor has a lot of complexity already, so if we can avoid adding another 100 lines of code here I'd prefer it, personally...

(Inserting a specific, concrete inline widget instance into an Editor does feel like a core function of Editor OTOH -- but that already is in this file: `addInlineWidget()`).