Port to the new wallpaper api

the root object must be a WallpaperItem
the fake public api setUrl become a signal handler
new actions api identical to the plasmoid one

Adds a small feature in potd: context action to open the downloaded photo of the day to save it

in hunyango, context action to trigger the random color animation that happens every now and then

needs https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/824