Use more colums in ::choose_string().

  We previously limited our choices to four colums, now we'll use
 up to twelve.  The following is my test-code:

    choose_string( "a", "b", "c", "d", "e" , "f" .. )

  We'll make sure we limit the number of colums to <= the size of
 the selection though, to avoid empty columns which looks weird.
  The enum is private.