stack: Don't place Wayland popups in o-r layer

Merged Jonas Ådahl requested to merge jadahl/mutter:wip/popup-not-in-o-r-layer into master

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.

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