Show all folders in backgrounds directories in XDG data dirs in wallpaper folder selector

In the "Background" tab of the "Desktop" settings section, pressing on the "Folder" thing near the bottom shows some folders from which the user can select to pick a wallpaper. Screenshot:

It would be nice if that folder selector automatically included the directories in /usr/share/backgrounds; for instance, on my Chimera Linux system, in that directory I have chimera, xfce, and gnome subdirectories. While I have enough technical knowledge to know that /usr/share/backgrounds is where backgrounds from system packages are usually installed, many people using Xfce would not have that knowledge.

On Chimera Linux, this is particularly important because we patch the default wallpaper to a path in /usr/share/backgrounds/chimera; therefore, if somebody wanted to instead use a wallpaper from Xfce's offerings, they would have to manually select "Other..." in the selector then navigate to /usr/share/backgrounds/xfce.

The logic for this feature would probably involve checking for a backgrounds directory in each path in the XDG_DATA_DIRS environment variable and then showing each subdirectory. There are also some complications that probably need to be dealt with here (e.g. what about two subdirectories of two different backgrounds directories, which happen to share the same name).

xfdesktop version is 4.18.1.