wlr-layer-shell-v1: destroy xdg popups on unmap
Created by: RedSoxFan
Mirrors #1530 (xdg-shell) for wlr-layer-shell-v1 Supersedes/Closes #1418 (#1530 + this is basically #1418) Fixes swaywm/sway#4477
This destroys the xdg popups associated with a layer surface when the layer surface is unmapped. It does not make sense to keep the popups open when unmapped.