Fix unit test failure

Fix unit test failure

The recent commit 022b564831e2295ee3bb6e53ece53191b40d894a, which added
a few source files to kitemlistcontrollertest, caused a crash in that
test - an assert was hit because now a different virtual method was
called in one place (KStandardItemListView::initializeItemListWidget()),
and qobject_cast could not cast the 'item', which is of type
KFileItemListWidget, to its base class KStandardItemListWidget. Adding
the source file kfileitemlistwidget.cpp fixes this.

CCMAIL: ps_ml@gmx.de