Display and distinguish GNOME XML wallpaper collections

[cassidyjames]

GNOME supports an XML format for wallpaper collections that includes nice features like slideshows and time-based wallpaper changes. We even use this format from the contract if you select multiple images and choose to set them as a wallpaper. However, we don't distinguish these wallpapers in the settings in any way, making it difficult or impossible to set them again.

We will also need this to support synchronizing a light/dark wallpaper with the dark style preference, as that is likely to be a feature added to GNOME as well.

We could add a relevant symbolic badge to the bottom-right with a tooltip describing the type of wallpaper collection.

For light/dark style wallpapers, we could use a new "style" icon (like object-inverse-symbolic, which is the metaphor Android uses for Dark Mode). Here's a rough mockup of how we could distinguish these:

For slideshows, we could use either a symbolic stack of photos icon or a timer icon and a tooltip like, "Slideshow of 6 images"