Groups should allow adding existing windows

[txtyash]

Right now only newly spawned windows get added to the window group in the dwindle layout but pre existing windows can be added to the group with a nice behavior

Let's say I have a grouped window and on its left another window I'd like to add to the group then I should be able to do it with something like mod+shift+h, similarly to merge a window on the right of the group press mod+shift+l. This behavior makes grouping convenient and useful. I used to use it in xmonad.

[MightyPlaza]

Samerino