* Bring back /usr/share/wallpapers/postmarketos.jpg, which is used in at
least as the default in sway, mate, lxqt, xfce4
* Move postmarketos-gnome.xml here (instead of taking the version from
artwork.git), because the paths mentioned in the xml file are also set
in this package in pmaports. This makes it easier to maintain.
* Fix the paths in postmarketos-gnome.xml (postmarketos vs
postmarketOS).
* Keep the postmarketos-gnome.xml name, as we had it before recent
wallpaper changes, and as it is the same name in v23.12 still. This
should make it backwards compatible for people upgrading from v23.12.
* Move 10_pmOS-wallpaper.gschema.override here for the same reason.
Fixes: 72357ecf ("main/postmarketos-artwork: upgrade to Meadow wallpaper and make Plasma-compatible (MR 4805)")
[ci:skip-build]: already built successfully in CI
