Allow individual windows to be stacked

Right now the only way to have a floating window that can be moved, resized, and typed into (while not being part of any frame) is to have that window be stacked. Unfortunately, individual windows can't be stacked without stacking every window in the entire group.

It would be great if you could make individual windows stackable, while letting the rest of the windows in the group remain unstacked. This would make a lot of applications that interface through dialog windows usable (applications like kdialog, zenity, awmenu, and gmrun).