Revert previous commits: Just set focusser and search inputs to "readonly" instead.

 * don't rely on "display:none" for .select2-search-hidden as focussing the hidden input would cause an error in IE (ref. http://api.jquery.com/focus/)
 * instead of littering the source with additional conditions, just set "readonly" once if "minimumResultsForSearch" is set to a negative value