shell: Store parent–child links between shsurfs for window stacking

shell: Store parent–child links between shsurfs for window stacking

This ensures transient surfaces are included in the layer of their
parent, even if the parent later changes layers. It achieves this by
recursively changing the layers of all children of a surface when that
surface’s layer is changed. The recursion is unbounded unless transient
surfaces are restricted to not being children of other popup or transient
surfaces.

This fixes a bug whereby a surface which is transient for a fullscreen
surface could end up being stacked below it.

https://bugs.freedesktop.org/show_bug.cgi?id=69443