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.