Ensure that KItemListViewLayouter always has a size hint resolver

KItemListViewLayouter uses a KItemListSizeHintResolver to find out how
much space the items will need in the view.

Before this commit, the size hint resolver object could be changed at
runtime, and it could also be null. However, we never made use of these
possibilities, so all the code that checks if m_sizeHintResolver is
null is actually not needed at all.