Two extra fields now always inserted when SortItems are initialised

These are FieldSortSpecial (used to force certain items like ".." to the
top or the bottom of the list) and FieldFolder (used to ensure folders are
separated from files). These are required in addition to whatever the
primary sort variable is.

Obviously this will impact the speedup of the rest of the patch, but since
neither are string fields, this should be minimal.