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.