Merge branch 'add-pop-wallpapers' into 'master'

Add the PopOS wallpapers recipe

See merge request !366

recipes/artwork/pop-wallpapers/recipe.toml

+[source]
+git = "https://github.com/pop-os/wallpapers"
+[build]
+template = "custom"
+script = """
+mkdir -pv "${COOKBOOK_STAGE}"/usr/share/wallpapers
+cp -rv "${COOKBOOK_SOURCE}"/original/* "${COOKBOOK_STAGE}"/usr/share/wallpapers
+"""