Fix the small issue of being able to clear the selection on a KPageView, if it is on list or tree view mode.
I reported a bug to TT, but it seems the fix was done on the docs QAbstractItemView::SingleSelection (check between 4.4.0 and 4.4.1).

Now that there is no hope that this will be done into Qt, I create our own selection model that will reject to clear the current selection if there is no other item going to be selected.