When the current item is removed, make -1 the current index temporarily

This fixes two problems:
1. KItemListKeyboardSearchManger can cancel the current search when a
   new folder is opened (note that this action removes the current item
   from the view).
2. The view can underline the new current item (which is the item that
   used to be below the removed item). Note that this did not work
   before because the view did not receive a currentChanged() signal in
   this case and therefore did not update the "current item" status of
   the new current item.

CCBUG: 297488
CCBUG: 298782
REVIEW: 104709
(cherry picked from commit 68ce395a)