Allow to stack two layer surfaces above/below each other

For phosh!1262 and phosh!1250 we want to fixate the surface ordering between the overview's / top-panel's background and the panel itself. This protocol extension allows for that. This is a bit easier than using subsurfaces since the later share the parent surfaces alpha value and we want to tweak them differently. Also they're relative to the parent surface while we want the backgrounds to be fixed position.