Adds support for layout level supplementary views.

[rafaelnobrekz]

This is kinda rudimentary support, but it suited my need for a global header at the layout configuration level. It also fixes a discrepancy on nested scrollview's subview ordering compared to the native implementation (which affected zIndex rendering of the section headers)

[rafaelnobrekz]

BTW for anyone interested, we forked and merged the outstanding PR's on the repo into our own