This function queries the bookmarks added by the user to the places sidebar,
and returns one of them. This function is used by #GtkFileChooser to implement
the “Alt-1”, “Alt-2”, etc. shortcuts, which activate the cooresponding bookmark.
The bookmark specified by the index @n, or
%NULL if no such index exist. Note that the indices start at 0, even though
the file chooser starts them with the keyboard shortcut "Alt-1".
This function queries the bookmarks added by the user to the places sidebar, and returns one of them. This function is used by #GtkFileChooser to implement the “Alt-1”, “Alt-2”, etc. shortcuts, which activate the cooresponding bookmark.