main/postmarketos-artwork: set plasma wallpaper
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.