Warn before overwriting another playlist on disk

When saving a playlist to an existing file on disk,
warn user, unless the playlist's file name remains
unchanged.

Change-Id: I10d82667de5fadb5323be4f981bea9263849f07a

apps/playlist_catalog.h

@@ -33,6 +33,9 @@ void catalog_set_directory(const char* directory);
  */
 bool catalog_view_playlists(void);
 
+bool catalog_pick_new_playlist_name(char *pl_name, size_t buf_size,
+                                    const char* curr_pl_name);
+
 /* 
  * Add something to a playlist (new or select from list of playlists in
  * catalog).