Add all background dirs to the settings chooser button shortcuts
This adds all directories found that match $XDG_DATA_DIRS/backgrounds/xfce/ or $XDG_DATA_DIRS/backgrounds/, if they have image files in them. It also adds the directory that holds DEFAULT_BACKDROP, if it's not in one of those found directories.
This isn't perfect, as GtkFileChooserButton doesn't detect when there are multiple shortcuts with the same last directory component, and try to disambiguate somehow. But I think that's probably ok, people can just check them out and not necessarily care where they are.
Closes #303 (closed)