Feature: add modes to create windows in vertical stack

[jtaala]

This PR improves the situation described in #858, and resolves #725;

More specifically, it adds new "create window" modes:

• down: create new windows in vertical stack (below current active window);
• up: create new windows in the vertical stack (above current active window);

This PR also adds new keybinds (unset by default) to directly set the position to open windows: