Make it compatible with regular autocomplete

I use widget factory extension for AutoComplete because I need to do globally some stuff everytime this plugin is used. But I also need MultiColumnAutocomplete capabilities available. If I changed extension to extend MultiColumnAutocomplete instead, it fails if I don't specify parameters this plugin is utilizing (e.g. columns). This patch allows to use these options only when needed and gracefully fallback to parent methods otherwise. I also improved it by removing not needed self and thead vars and replace code duplicated from parent method by calling parent instead.