Revert "Revert "Simplify vim window mappings""

So, splitright and splitbelow do work prefectly fine in MacVim, but my
session files contained nosplitright and nosplitbelow. TBH, I'm still
not sure how this happened, my sessionoptions do not include "options".

This reverts commit c718656a62dc274b64c5843ef3a126e3dabe00f8.