add type coercion helper to ListItem objects

src/playlist.rs

@@ -107,6 +107,10 @@ impl ListItem for Playlist {
         }
     }
 
+    fn as_listitem(&self) -> Box<ListItem> {
+        Box::new(self.clone())
+    }
+
     fn display_left(&self) -> String {
         self.name.clone()
     }