Add a layoutmsg param for preselecting split ratio in Dwindle layout

[ImYuugen]

We have the preselect layoutmsg for preselecting the next window's direction, but there is no way (that I'm aware of) to select a custom split ratio for the next window to be opened.

I do not have a PoC yet, a good equivalent is bspwm's preselections with bspc node -o <split ratio>

As an example:
Let's say I have a singular window opened in tiling mode, I want to open another one below, but make it take up only 20% of the screen.
I could set up my bindings like

bind = SUPER, Return, exec, $terminal
bind = SUPER ALT, j, layoutmsg, preselect d
bind = SUPER ALT, 2, layoutmsg, preselectratio 0.2

Then press SUPER ALT j, SUPER ALT 2, SUPER Return to have it spawn below the current one, taking up 20% of the space