Set the postmarketOS wallpaper as default for Plasma Mobile and Plasma
Desktop, so we can change it across all UIs with every postmarketOS
release.
It would be nice if we could just set the wallpaper with a simple config
file (reported https://bugs.kde.org/show_bug.cgi?id=487816). Until that
is possible, use a shell script as workaround.
Copy the contents dir and metadata.json file explicitly, instead of
copying the entire "meadow" directory. With this change we avoid copying
meadow.svg, which fixes two issues:
* This file doesn't have a stable name, when we switch to the next
wallpaper and the user had selected it, the wallpaper will just be
gone instead of automatically switching to the new wallpaper.
* It shows up as broken wallpaper in Plasma's wallpaper selection.
[ci:skip-build]: already built successfully in CI
