Reverse Span - Reduce size of windows at the edge of screen

**Describe the feature you'd like**

It would be nice to have: A way to revert a span or when the window can't span to an empty square (like if we are at the edge of the screen) it would reduce the size and span of the window in the direction of the edge.

Lets say I have a small window on the right edge on my screen. I span it to a square that is to the left, this works fine. But now, lets say I want to decrease the size of the window by one square. Basically return it to its previous state, I cant do that.

When I span the window to another square by my shortcut (WIN+CTRL+ but I realize that I have gone to far, and I want to go back, I intuitevly try to reverse the action by pressing the opposing direction arrow. If there would be an empty square there, the window would span to that square, but if there is no empty square, if the window is at the edge of the screen, nothing happends.

I would like to be able to keep pressing WIN+CTRL and if the window can't span to an empty square in the direction I'm pressing, I would like it to get smaller. Basically reducing the size of the window, by one square, in the direction I'm pressing.

What I can do already, is to move the window to a new empty square, and then span it again. Which works fine, but is a bit unintuitive.

Thank you for this great application! It really helps with my day to day work on big screens. ♥️

Start by tracing the WIN+CTRL+direction-arrow shortcut and the span operation in the window-management code. Compare behavior at a screen edge with an available empty square; done means opposing presses can reduce the window by one square when no further span is possible, without breaking existing spanning.

Written by the indexing model from the issue text.