Internal KFileItemModel optimizations and cleanups

Internal KFileItemModel optimizations and cleanups

- Use merge-sort instead of quick-sort. This assures a sane
  worst-case scenario where quick-sort has a runtime complexity of
  O(n*n) (e.g. when changing the sort-order from ascending to
  descending).

- lessThan()-improvements: Change internal data-structures to
  allow a comparison of any role, not only roles available
  in KFileItem

- Don't synchronously move an item if the value has been changed
  of a role defined as sort-role: This is too expensive in case if
  e.g. the sorting is done by "type" and the type is determined
  step by step.