... and use this to reduce the number of explicit uses of m_children.

Despite of being shorter code by itself it is a step towards having
an explicit LeafItem object that doesn't explicitly store a(n empty)
vector of child nodes.

Change-Id: If8db85e2f1134dd1578a78d31235bf57a28f863a
Reviewed-by: Eike Ziller <eike.ziller@qt.io>