- assign the original prefix to a comparator so it does not change
  when the user continues typing in the pattern text entry
- changed TypeItemsComparator constructor to take a TypeItemsFilter
  and use that filter to get the original pattern which is used
  to set the original prefix used for comparisons
- remove fTypeItemsComparator field and have a new TypeItemsComparator
  created each time getItemsComparator() and pass the fFilter field
- remove getPatternText() override method and replace with a new
  private method getModifiedPatternText() which applies the logic
  used previously but use this new method in TypeItemsFilter constructor
- fixes #2509