Add configuration option to control new window insertion direction

[nikitabobko]

Right now, all new windows are inserted right after the mru window (most recently used window). The feature proposal is to add a configuration option to insert new windows before the mru window.

# Possible values: after-the-mru-window|before-the-mru-window
default-windows-insertion-point = 'after-the-mru-window'

The only thing that I'm hesitant about is the naming. I'd love to avoid scaring people with mru term (though, it's the way we call it in the code base right now, and it's technically correct)

Reported by @fcomrqz in #1768

[jco-oneg]

I would also love to have something like this for my setup. As I always need to "pickup" and "clean after" new windows that are created.

I have a quick question: what does "after"/"before" entail?
Will the window be place in the same container as the mru? or will it create a opposite orientation container? Is it possible/simple to support both cases?