Use xdg-desktop-portal for setting Wallpaper

Merged Felipe Borges requested to merge use-wallpaper-portal into master

When libportal is available, we will relay the wallpaper setup to xdg-desktop-portal/wallpaper https://github.com/flatpak/xdg-desktop-portal/blob/master/data/org.freedesktop.impl.portal.Wallpaper.xml

When libportal is not found, we continue copying and setting the wallpaper image ourselves.

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