Reimplement name-filtering

The filtering of items has not been implemented yet in
the KFileItemModel of the new view-engine. The patch brings back
this functionality again, but some minor issues are open:
- When filtering trees expanded directories should only get
  hidden if no child is visible
- Regular expressions are not supported yet (they have not been
  supported in Dolphin 1.x but it is now quite simple to implement).
- When filtering previews and removing the filter it might be
  possible that the preview is not shown (is most probably an
  an already existing bug in KFileItemModelRolesUpdater).

BUG: 287642
FIXED-IN: 4.8.0