Enable caching of field values in a WordListBuilder for faster ListFieldValues calls

When using ListWords(), pad the top of the list with blank entries if there are not enough words before the searched word in the index. For example, after a call to ListWords("0", 5), if "0" (zero) is the first word in the index, then five blank words will be added to the top of the generated list. dtsWordListPadTopOfList only affects calls to WordListBuilder.ListWords().