General: allow the central widget to be split horizontally

[agarny]

This replaces issue #2507 which was about allowing multiple instances of OpenCOR. However, to allow this would result in various side effects, so it would be better to allow the central widget to be split horizontally. This is, in face, what Visual Studio Code does, i.e. only one instance which central widget can be split horizontally.

[agarny]

This is likely going to be quite a bit of work, so moving this to our 0.8 milestone.