Section state

Section state lives as a node of the section graph. As the section graph is heterogeneous, the section state has children that are either items or other section states, however, this difference is hidden from outside observers. The section state also functions as an interface for the Slm to query the section and to access child items. To avoid bloating the section state further, layout state is separated out as a property, and treated as a separate thing by the Slm.