Interface cleanups to prepare the return of "grouped sorting"

Interface cleanups to prepare the return of "grouped sorting"

- Rename setCategorizedSorting() to setGroupedSorting()
- Change the model interface to allow enabling/disabling grouping
  without the need to declare a role (the sort role will be taken).
- Add dummy group role implementation in KFileItemModel

The grouping code itself requires some cleanups and might crash
at the moment or lead to weird layouts.