Implement directional window splitting

[Tyriar]

While VSCode allows splitting of windows to the left using ctrl+</kbd>, Atom allows splitting in 4 directions with the shortcut ctrl+k, <arrow>. The main advantage this has over the current workbench.action.splitEditor command is that it enables splitting the editor to allow both horizontal and vertical layouts.

Note: VSCode currently uses ctrl+k, up and ctrl+k, down for opening the previous and next working file.

[AndrewNatoli]

I agree horizontal splitting would be awesome :D

It looks like this is being worked on for the debug / console output panels. Would be nice to eventually see it for editor panels

[joaomoreno]

@stevencl @bgashler1 @bpasero not sure if there's a dupe for this already

[bpasero]

#1749