Now that there's only a single background to set, there's no use for a flag parameter to specify the background(s). https://gitlab.gnome.org/GNOME/gnome-control-center/issues/864
This makes the new Background panel feature-complete; we can add and remove custom wallpapers now.
It will become essentially what CcBackgroundChooserDialog, but without a dialog and organized differently. Right now, it is as minimal as it could be and only shows wallpapers. As we grow it, it may cover pictures and even colors.