Dwindle: add ability to move pseudo-tiled windows inside their dedicated space

[seijileroux]

I'm looking forward to having the ability to arrange pseudo-tiled windows inside their dedicated space box. Ideally, some command that could be bound to up-down-left-right keybinds.

Here's an example to demonstrate what I mean.
Say, I have a vertical screen with a pseudo-tiled mpv player on it. Naturally, it gets aligned in the middle of the screen, but what I'd like to be able to do is to move it all the way to the top, leaving the rest of the screen underneath an empty space:

So far, I can supplement it by adding a 2nd window underneath, like a terminal:

But not having a natural way of controlling window position in this case seems like an oversight for a pseudo feature preventing more people from adopting it in their daily workflow.

P.S. I've also considered using a floating mpv window instead, but that concept becomes inapplicable in any case of having 2 or more windows on the workspace, as after popping out the floating window will shift the layout of all other windows to take mpv's space.