[REVIEW_ONLY] Splitview

[JeffryBooher]

• 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:
#8513 (comment)
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.

[JeffryBooher]

Closing in lieu of #8913