Add a check for invisible_char validity
    
    The invisible_char setting is usually non-ASCII, so sometimes, say if
    someone opens a glade file in a non-unicode-aware editor and then saves
    the file, the string gets trashed to ? or ???. Check that this didn't
    happen.
    
    Also add a check that invisible_char is needed at all since it's just
    glade noise more often than not.