stack: Don't place Wayland popups in o-r layer
Placing persistant Wayland popups (e.g. not menus etc) in the o-r layer breaks stacking order with other window trees (e.g. other client windows), as the menu would get stuck in the o-r layer, i.e. on top, even if the window the parent of the popup got lowered.
Fix this by placing the popups in the normal layer, relying on transient-ness to keep stacking correct.