Allow splitting the editor vertically instead of horizontally

This would really help when working side-by-side with another app on a smaller monitor.

By "splitting", I mean having the editor on top and the preview on bottom (or the opposite).

Great work with your app, I love it!

Start at the editor and preview layout entry points and determine how the current horizontal split is assembled. Done means users can choose a vertical editor/preview arrangement, with the existing horizontal arrangement still available and both panes usable on smaller monitors.

Written by the indexing model from the issue text.