CRM-13303: Views doesn't handle string fields like strings

Views has an argument handler for strings that contains a number of useful tools, most notably "Glossary Mode" for using the argument "A" to return all values starting with that letter.  However, practically none of the CiviCRM string fields use that argument handler.  This makes it difficult to do a member directory, event listing, or other view with an index on the letter.

The fix is simply changing the handler to views_handler_argument_string instead of views_handler_argument.