desk.cpl: Include files from the wallpaper directory in the list of available desktop backgrounds

The patch moves the code parts that enumerate available wallpaper files from AddListViewItems into
a separate function, that can be used to add the images found in any given directory to the list of
available background images and adds two calls to it to AddListViewItems, one for the directory
given by GetWindowsDirectory and one for the wallpaper directory as given in the registry.