BListView: Fix missing update for list items.

- According to the Be Book, one of BListItem::Update()'s responsibilities
  is updating the item's internal height/width. However, while we were
  providing such updates in the case of the item initially being added
  to the view, or when the list view's font was changed, we were not doing
  so on frame resize. This would lead to items having a stale width field
  in such a circumstance, leading to possibly incorrect behavior in apps
  relying on that field to be correct.

src/kits/interface/ListView.cpp

@@ -257,6 +257,9 @@ void
 BListView::FrameResized(float newWidth, float newHeight)
 {
 	_FixupScrollBar();
+
+	// notify items of new width.
+	_FontChanged();
 }